Merge tag 'docs-6.1-2' of git://git.lwn.net/linux

[platform/kernel/linux-starfive.git] / mm / gup.c

// SPDX-License-Identifier: GPL-2.0-only
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/err.h>
#include <linux/spinlock.h>

#include <linux/mm.h>
#include <linux/memremap.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/secretmem.h>

#include <linux/sched/signal.h>
#include <linux/rwsem.h>
#include <linux/hugetlb.h>
#include <linux/migrate.h>
#include <linux/mm_inline.h>
#include <linux/sched/mm.h>

#include <asm/mmu_context.h>
#include <asm/tlbflush.h>

#include "internal.h"

struct follow_page_context {
        struct dev_pagemap *pgmap;
        unsigned int page_mask;
};

static inline void sanity_check_pinned_pages(struct page **pages,
                                             unsigned long npages)
{
        if (!IS_ENABLED(CONFIG_DEBUG_VM))
                return;

        /*
         * We only pin anonymous pages if they are exclusive. Once pinned, we
         * can no longer turn them possibly shared and PageAnonExclusive() will
         * stick around until the page is freed.
         *
         * We'd like to verify that our pinned anonymous pages are still mapped
         * exclusively. The issue with anon THP is that we don't know how
         * they are/were mapped when pinning them. However, for anon
         * THP we can assume that either the given page (PTE-mapped THP) or
         * the head page (PMD-mapped THP) should be PageAnonExclusive(). If
         * neither is the case, there is certainly something wrong.
         */
        for (; npages; npages--, pages++) {
                struct page *page = *pages;
                struct folio *folio = page_folio(page);

                if (!folio_test_anon(folio))
                        continue;
                if (!folio_test_large(folio) || folio_test_hugetlb(folio))
                        VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
                else
                        /* Either a PTE-mapped or a PMD-mapped THP. */
                        VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page) &&
                                       !PageAnonExclusive(page), page);
        }
}

/*
 * Return the folio with ref appropriately incremented,
 * or NULL if that failed.
 */
static inline struct folio *try_get_folio(struct page *page, int refs)
{
        struct folio *folio;

retry:
        folio = page_folio(page);
        if (WARN_ON_ONCE(folio_ref_count(folio) < 0))
                return NULL;
        if (unlikely(!folio_ref_try_add_rcu(folio, refs)))
                return NULL;

        /*
         * At this point we have a stable reference to the folio; but it
         * could be that between calling page_folio() and the refcount
         * increment, the folio was split, in which case we'd end up
         * holding a reference on a folio that has nothing to do with the page
         * we were given anymore.
         * So now that the folio is stable, recheck that the page still
         * belongs to this folio.
         */
        if (unlikely(page_folio(page) != folio)) {
                if (!put_devmap_managed_page_refs(&folio->page, refs))
                        folio_put_refs(folio, refs);
                goto retry;
        }

        return folio;
}

/**
 * try_grab_folio() - Attempt to get or pin a folio.
 * @page:  pointer to page to be grabbed
 * @refs:  the value to (effectively) add to the folio's refcount
 * @flags: gup flags: these are the FOLL_* flag values.
 *
 * "grab" names in this file mean, "look at flags to decide whether to use
 * FOLL_PIN or FOLL_GET behavior, when incrementing the folio's refcount.
 *
 * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
 * same time. (That's true throughout the get_user_pages*() and
 * pin_user_pages*() APIs.) Cases:
 *
 *    FOLL_GET: folio's refcount will be incremented by @refs.
 *
 *    FOLL_PIN on large folios: folio's refcount will be incremented by
 *    @refs, and its compound_pincount will be incremented by @refs.
 *
 *    FOLL_PIN on single-page folios: folio's refcount will be incremented by
 *    @refs * GUP_PIN_COUNTING_BIAS.
 *
 * Return: The folio containing @page (with refcount appropriately
 * incremented) for success, or NULL upon failure. If neither FOLL_GET
 * nor FOLL_PIN was set, that's considered failure, and furthermore,
 * a likely bug in the caller, so a warning is also emitted.
 */
struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
{
        if (flags & FOLL_GET)
                return try_get_folio(page, refs);
        else if (flags & FOLL_PIN) {
                struct folio *folio;

                /*
                 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
                 * right zone, so fail and let the caller fall back to the slow
                 * path.
                 */
                if (unlikely((flags & FOLL_LONGTERM) &&
                             !is_longterm_pinnable_page(page)))
                        return NULL;

                /*
                 * CAUTION: Don't use compound_head() on the page before this
                 * point, the result won't be stable.
                 */
                folio = try_get_folio(page, refs);
                if (!folio)
                        return NULL;

                /*
                 * When pinning a large folio, use an exact count to track it.
                 *
                 * However, be sure to *also* increment the normal folio
                 * refcount field at least once, so that the folio really
                 * is pinned.  That's why the refcount from the earlier
                 * try_get_folio() is left intact.
                 */
                if (folio_test_large(folio))
                        atomic_add(refs, folio_pincount_ptr(folio));
                else
                        folio_ref_add(folio,
                                        refs * (GUP_PIN_COUNTING_BIAS - 1));
                /*
                 * Adjust the pincount before re-checking the PTE for changes.
                 * This is essentially a smp_mb() and is paired with a memory
                 * barrier in page_try_share_anon_rmap().
                 */
                smp_mb__after_atomic();

                node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);

                return folio;
        }

        WARN_ON_ONCE(1);
        return NULL;
}

static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
{
        if (flags & FOLL_PIN) {
                node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
                if (folio_test_large(folio))
                        atomic_sub(refs, folio_pincount_ptr(folio));
                else
                        refs *= GUP_PIN_COUNTING_BIAS;
        }

        if (!put_devmap_managed_page_refs(&folio->page, refs))
                folio_put_refs(folio, refs);
}

/**
 * try_grab_page() - elevate a page's refcount by a flag-dependent amount
 * @page:    pointer to page to be grabbed
 * @flags:   gup flags: these are the FOLL_* flag values.
 *
 * This might not do anything at all, depending on the flags argument.
 *
 * "grab" names in this file mean, "look at flags to decide whether to use
 * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
 *
 * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
 * time. Cases: please see the try_grab_folio() documentation, with
 * "refs=1".
 *
 * Return: true for success, or if no action was required (if neither FOLL_PIN
 * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
 * FOLL_PIN was set, but the page could not be grabbed.
 */
bool __must_check try_grab_page(struct page *page, unsigned int flags)
{
        struct folio *folio = page_folio(page);

        WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
        if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
                return false;

        if (flags & FOLL_GET)
                folio_ref_inc(folio);
        else if (flags & FOLL_PIN) {
                /*
                 * Similar to try_grab_folio(): be sure to *also*
                 * increment the normal page refcount field at least once,
                 * so that the page really is pinned.
                 */
                if (folio_test_large(folio)) {
                        folio_ref_add(folio, 1);
                        atomic_add(1, folio_pincount_ptr(folio));
                } else {
                        folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
                }

                node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
        }

        return true;
}

/**
 * unpin_user_page() - release a dma-pinned page
 * @page:            pointer to page to be released
 *
 * Pages that were pinned via pin_user_pages*() must be released via either
 * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
 * that such pages can be separately tracked and uniquely handled. In
 * particular, interactions with RDMA and filesystems need special handling.
 */
void unpin_user_page(struct page *page)
{
        sanity_check_pinned_pages(&page, 1);
        gup_put_folio(page_folio(page), 1, FOLL_PIN);
}
EXPORT_SYMBOL(unpin_user_page);

static inline struct folio *gup_folio_range_next(struct page *start,
                unsigned long npages, unsigned long i, unsigned int *ntails)
{
        struct page *next = nth_page(start, i);
        struct folio *folio = page_folio(next);
        unsigned int nr = 1;

        if (folio_test_large(folio))
                nr = min_t(unsigned int, npages - i,
                           folio_nr_pages(folio) - folio_page_idx(folio, next));

        *ntails = nr;
        return folio;
}

static inline struct folio *gup_folio_next(struct page **list,
                unsigned long npages, unsigned long i, unsigned int *ntails)
{
        struct folio *folio = page_folio(list[i]);
        unsigned int nr;

        for (nr = i + 1; nr < npages; nr++) {
                if (page_folio(list[nr]) != folio)
                        break;
        }

        *ntails = nr - i;
        return folio;
}

/**
 * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
 * @pages:  array of pages to be maybe marked dirty, and definitely released.
 * @npages: number of pages in the @pages array.
 * @make_dirty: whether to mark the pages dirty
 *
 * "gup-pinned page" refers to a page that has had one of the get_user_pages()
 * variants called on that page.
 *
 * For each page in the @pages array, make that page (or its head page, if a
 * compound page) dirty, if @make_dirty is true, and if the page was previously
 * listed as clean. In any case, releases all pages using unpin_user_page(),
 * possibly via unpin_user_pages(), for the non-dirty case.
 *
 * Please see the unpin_user_page() documentation for details.
 *
 * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
 * required, then the caller should a) verify that this is really correct,
 * because _lock() is usually required, and b) hand code it:
 * set_page_dirty_lock(), unpin_user_page().
 *
 */
void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
                                 bool make_dirty)
{
        unsigned long i;
        struct folio *folio;
        unsigned int nr;

        if (!make_dirty) {
                unpin_user_pages(pages, npages);
                return;
        }

        sanity_check_pinned_pages(pages, npages);
        for (i = 0; i < npages; i += nr) {
                folio = gup_folio_next(pages, npages, i, &nr);
                /*
                 * Checking PageDirty at this point may race with
                 * clear_page_dirty_for_io(), but that's OK. Two key
                 * cases:
                 *
                 * 1) This code sees the page as already dirty, so it
                 * skips the call to set_page_dirty(). That could happen
                 * because clear_page_dirty_for_io() called
                 * page_mkclean(), followed by set_page_dirty().
                 * However, now the page is going to get written back,
                 * which meets the original intention of setting it
                 * dirty, so all is well: clear_page_dirty_for_io() goes
                 * on to call TestClearPageDirty(), and write the page
                 * back.
                 *
                 * 2) This code sees the page as clean, so it calls
                 * set_page_dirty(). The page stays dirty, despite being
                 * written back, so it gets written back again in the
                 * next writeback cycle. This is harmless.
                 */
                if (!folio_test_dirty(folio)) {
                        folio_lock(folio);
                        folio_mark_dirty(folio);
                        folio_unlock(folio);
                }
                gup_put_folio(folio, nr, FOLL_PIN);
        }
}
EXPORT_SYMBOL(unpin_user_pages_dirty_lock);

/**
 * unpin_user_page_range_dirty_lock() - release and optionally dirty
 * gup-pinned page range
 *
 * @page:  the starting page of a range maybe marked dirty, and definitely released.
 * @npages: number of consecutive pages to release.
 * @make_dirty: whether to mark the pages dirty
 *
 * "gup-pinned page range" refers to a range of pages that has had one of the
 * pin_user_pages() variants called on that page.
 *
 * For the page ranges defined by [page .. page+npages], make that range (or
 * its head pages, if a compound page) dirty, if @make_dirty is true, and if the
 * page range was previously listed as clean.
 *
 * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
 * required, then the caller should a) verify that this is really correct,
 * because _lock() is usually required, and b) hand code it:
 * set_page_dirty_lock(), unpin_user_page().
 *
 */
void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
                                      bool make_dirty)
{
        unsigned long i;
        struct folio *folio;
        unsigned int nr;

        for (i = 0; i < npages; i += nr) {
                folio = gup_folio_range_next(page, npages, i, &nr);
                if (make_dirty && !folio_test_dirty(folio)) {
                        folio_lock(folio);
                        folio_mark_dirty(folio);
                        folio_unlock(folio);
                }
                gup_put_folio(folio, nr, FOLL_PIN);
        }
}
EXPORT_SYMBOL(unpin_user_page_range_dirty_lock);

static void unpin_user_pages_lockless(struct page **pages, unsigned long npages)
{
        unsigned long i;
        struct folio *folio;
        unsigned int nr;

        /*
         * Don't perform any sanity checks because we might have raced with
         * fork() and some anonymous pages might now actually be shared --
         * which is why we're unpinning after all.
         */
        for (i = 0; i < npages; i += nr) {
                folio = gup_folio_next(pages, npages, i, &nr);
                gup_put_folio(folio, nr, FOLL_PIN);
        }
}

/**
 * unpin_user_pages() - release an array of gup-pinned pages.
 * @pages:  array of pages to be marked dirty and released.
 * @npages: number of pages in the @pages array.
 *
 * For each page in the @pages array, release the page using unpin_user_page().
 *
 * Please see the unpin_user_page() documentation for details.
 */
void unpin_user_pages(struct page **pages, unsigned long npages)
{
        unsigned long i;
        struct folio *folio;
        unsigned int nr;

        /*
         * If this WARN_ON() fires, then the system *might* be leaking pages (by
         * leaving them pinned), but probably not. More likely, gup/pup returned
         * a hard -ERRNO error to the caller, who erroneously passed it here.
         */
        if (WARN_ON(IS_ERR_VALUE(npages)))
                return;

        sanity_check_pinned_pages(pages, npages);
        for (i = 0; i < npages; i += nr) {
                folio = gup_folio_next(pages, npages, i, &nr);
                gup_put_folio(folio, nr, FOLL_PIN);
        }
}
EXPORT_SYMBOL(unpin_user_pages);

/*
 * Set the MMF_HAS_PINNED if not set yet; after set it'll be there for the mm's
 * lifecycle.  Avoid setting the bit unless necessary, or it might cause write
 * cache bouncing on large SMP machines for concurrent pinned gups.
 */
static inline void mm_set_has_pinned_flag(unsigned long *mm_flags)
{
        if (!test_bit(MMF_HAS_PINNED, mm_flags))
                set_bit(MMF_HAS_PINNED, mm_flags);
}

#ifdef CONFIG_MMU
static struct page *no_page_table(struct vm_area_struct *vma,
                unsigned int flags)
{
        /*
         * When core dumping an enormous anonymous area that nobody
         * has touched so far, we don't want to allocate unnecessary pages or
         * page tables.  Return error instead of NULL to skip handle_mm_fault,
         * then get_dump_page() will return NULL to leave a hole in the dump.
         * But we can only make this optimization where a hole would surely
         * be zero-filled if handle_mm_fault() actually did handle it.
         */
        if ((flags & FOLL_DUMP) &&
                        (vma_is_anonymous(vma) || !vma->vm_ops->fault))
                return ERR_PTR(-EFAULT);
        return NULL;
}

static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
                pte_t *pte, unsigned int flags)
{
        if (flags & FOLL_TOUCH) {
                pte_t entry = *pte;

                if (flags & FOLL_WRITE)
                        entry = pte_mkdirty(entry);
                entry = pte_mkyoung(entry);

                if (!pte_same(*pte, entry)) {
                        set_pte_at(vma->vm_mm, address, pte, entry);
                        update_mmu_cache(vma, address, pte);
                }
        }

        /* Proper page table entry exists, but no corresponding struct page */
        return -EEXIST;
}

/* FOLL_FORCE can write to even unwritable PTEs in COW mappings. */
static inline bool can_follow_write_pte(pte_t pte, struct page *page,
                                        struct vm_area_struct *vma,
                                        unsigned int flags)
{
        /* If the pte is writable, we can write to the page. */
        if (pte_write(pte))
                return true;

        /* Maybe FOLL_FORCE is set to override it? */
        if (!(flags & FOLL_FORCE))
                return false;

        /* But FOLL_FORCE has no effect on shared mappings */
        if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
                return false;

        /* ... or read-only private ones */
        if (!(vma->vm_flags & VM_MAYWRITE))
                return false;

        /* ... or already writable ones that just need to take a write fault */
        if (vma->vm_flags & VM_WRITE)
                return false;

        /*
         * See can_change_pte_writable(): we broke COW and could map the page
         * writable if we have an exclusive anonymous page ...
         */
        if (!page || !PageAnon(page) || !PageAnonExclusive(page))
                return false;

        /* ... and a write-fault isn't required for other reasons. */
        if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte))
                return false;
        return !userfaultfd_pte_wp(vma, pte);
}

static struct page *follow_page_pte(struct vm_area_struct *vma,
                unsigned long address, pmd_t *pmd, unsigned int flags,
                struct dev_pagemap **pgmap)
{
        struct mm_struct *mm = vma->vm_mm;
        struct page *page;
        spinlock_t *ptl;
        pte_t *ptep, pte;
        int ret;

        /* FOLL_GET and FOLL_PIN are mutually exclusive. */
        if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
                         (FOLL_PIN | FOLL_GET)))
                return ERR_PTR(-EINVAL);

        /*
         * Considering PTE level hugetlb, like continuous-PTE hugetlb on
         * ARM64 architecture.
         */
        if (is_vm_hugetlb_page(vma)) {
                page = follow_huge_pmd_pte(vma, address, flags);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }

retry:
        if (unlikely(pmd_bad(*pmd)))
                return no_page_table(vma, flags);

        ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
        pte = *ptep;
        if (!pte_present(pte)) {
                swp_entry_t entry;
                /*
                 * KSM's break_ksm() relies upon recognizing a ksm page
                 * even while it is being migrated, so for that case we
                 * need migration_entry_wait().
                 */
                if (likely(!(flags & FOLL_MIGRATION)))
                        goto no_page;
                if (pte_none(pte))
                        goto no_page;
                entry = pte_to_swp_entry(pte);
                if (!is_migration_entry(entry))
                        goto no_page;
                pte_unmap_unlock(ptep, ptl);
                migration_entry_wait(mm, pmd, address);
                goto retry;
        }
        if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
                goto no_page;

        page = vm_normal_page(vma, address, pte);

        /*
         * We only care about anon pages in can_follow_write_pte() and don't
         * have to worry about pte_devmap() because they are never anon.
         */
        if ((flags & FOLL_WRITE) &&
            !can_follow_write_pte(pte, page, vma, flags)) {
                page = NULL;
                goto out;
        }

        if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
                /*
                 * Only return device mapping pages in the FOLL_GET or FOLL_PIN
                 * case since they are only valid while holding the pgmap
                 * reference.
                 */
                *pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);
                if (*pgmap)
                        page = pte_page(pte);
                else
                        goto no_page;
        } else if (unlikely(!page)) {
                if (flags & FOLL_DUMP) {
                        /* Avoid special (like zero) pages in core dumps */
                        page = ERR_PTR(-EFAULT);
                        goto out;
                }

                if (is_zero_pfn(pte_pfn(pte))) {
                        page = pte_page(pte);
                } else {
                        ret = follow_pfn_pte(vma, address, ptep, flags);
                        page = ERR_PTR(ret);
                        goto out;
                }
        }

        if (!pte_write(pte) && gup_must_unshare(flags, page)) {
                page = ERR_PTR(-EMLINK);
                goto out;
        }

        VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
                       !PageAnonExclusive(page), page);

        /* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
        if (unlikely(!try_grab_page(page, flags))) {
                page = ERR_PTR(-ENOMEM);
                goto out;
        }
        /*
         * We need to make the page accessible if and only if we are going
         * to access its content (the FOLL_PIN case).  Please see
         * Documentation/core-api/pin_user_pages.rst for details.
         */
        if (flags & FOLL_PIN) {
                ret = arch_make_page_accessible(page);
                if (ret) {
                        unpin_user_page(page);
                        page = ERR_PTR(ret);
                        goto out;
                }
        }
        if (flags & FOLL_TOUCH) {
                if ((flags & FOLL_WRITE) &&
                    !pte_dirty(pte) && !PageDirty(page))
                        set_page_dirty(page);
                /*
                 * pte_mkyoung() would be more correct here, but atomic care
                 * is needed to avoid losing the dirty bit: it is easier to use
                 * mark_page_accessed().
                 */
                mark_page_accessed(page);
        }
out:
        pte_unmap_unlock(ptep, ptl);
        return page;
no_page:
        pte_unmap_unlock(ptep, ptl);
        if (!pte_none(pte))
                return NULL;
        return no_page_table(vma, flags);
}

static struct page *follow_pmd_mask(struct vm_area_struct *vma,
                                    unsigned long address, pud_t *pudp,
                                    unsigned int flags,
                                    struct follow_page_context *ctx)
{
        pmd_t *pmd, pmdval;
        spinlock_t *ptl;
        struct page *page;
        struct mm_struct *mm = vma->vm_mm;

        pmd = pmd_offset(pudp, address);
        /*
         * The READ_ONCE() will stabilize the pmdval in a register or
         * on the stack so that it will stop changing under the code.
         */
        pmdval = READ_ONCE(*pmd);
        if (pmd_none(pmdval))
                return no_page_table(vma, flags);
        if (pmd_huge(pmdval) && is_vm_hugetlb_page(vma)) {
                page = follow_huge_pmd_pte(vma, address, flags);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        if (is_hugepd(__hugepd(pmd_val(pmdval)))) {
                page = follow_huge_pd(vma, address,
                                      __hugepd(pmd_val(pmdval)), flags,
                                      PMD_SHIFT);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
retry:
        if (!pmd_present(pmdval)) {
                /*
                 * Should never reach here, if thp migration is not supported;
                 * Otherwise, it must be a thp migration entry.
                 */
                VM_BUG_ON(!thp_migration_supported() ||
                                  !is_pmd_migration_entry(pmdval));

                if (likely(!(flags & FOLL_MIGRATION)))
                        return no_page_table(vma, flags);

                pmd_migration_entry_wait(mm, pmd);
                pmdval = READ_ONCE(*pmd);
                /*
                 * MADV_DONTNEED may convert the pmd to null because
                 * mmap_lock is held in read mode
                 */
                if (pmd_none(pmdval))
                        return no_page_table(vma, flags);
                goto retry;
        }
        if (pmd_devmap(pmdval)) {
                ptl = pmd_lock(mm, pmd);
                page = follow_devmap_pmd(vma, address, pmd, flags, &ctx->pgmap);
                spin_unlock(ptl);
                if (page)
                        return page;
        }
        if (likely(!pmd_trans_huge(pmdval)))
                return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);

        if (pmd_protnone(pmdval) && !gup_can_follow_protnone(flags))
                return no_page_table(vma, flags);

retry_locked:
        ptl = pmd_lock(mm, pmd);
        if (unlikely(pmd_none(*pmd))) {
                spin_unlock(ptl);
                return no_page_table(vma, flags);
        }
        if (unlikely(!pmd_present(*pmd))) {
                spin_unlock(ptl);
                if (likely(!(flags & FOLL_MIGRATION)))
                        return no_page_table(vma, flags);
                pmd_migration_entry_wait(mm, pmd);
                goto retry_locked;
        }
        if (unlikely(!pmd_trans_huge(*pmd))) {
                spin_unlock(ptl);
                return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
        }
        if (flags & FOLL_SPLIT_PMD) {
                int ret;
                page = pmd_page(*pmd);
                if (is_huge_zero_page(page)) {
                        spin_unlock(ptl);
                        ret = 0;
                        split_huge_pmd(vma, pmd, address);
                        if (pmd_trans_unstable(pmd))
                                ret = -EBUSY;
                } else {
                        spin_unlock(ptl);
                        split_huge_pmd(vma, pmd, address);
                        ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
                }

                return ret ? ERR_PTR(ret) :
                        follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
        }
        page = follow_trans_huge_pmd(vma, address, pmd, flags);
        spin_unlock(ptl);
        ctx->page_mask = HPAGE_PMD_NR - 1;
        return page;
}

static struct page *follow_pud_mask(struct vm_area_struct *vma,
                                    unsigned long address, p4d_t *p4dp,
                                    unsigned int flags,
                                    struct follow_page_context *ctx)
{
        pud_t *pud;
        spinlock_t *ptl;
        struct page *page;
        struct mm_struct *mm = vma->vm_mm;

        pud = pud_offset(p4dp, address);
        if (pud_none(*pud))
                return no_page_table(vma, flags);
        if (pud_huge(*pud) && is_vm_hugetlb_page(vma)) {
                page = follow_huge_pud(mm, address, pud, flags);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        if (is_hugepd(__hugepd(pud_val(*pud)))) {
                page = follow_huge_pd(vma, address,
                                      __hugepd(pud_val(*pud)), flags,
                                      PUD_SHIFT);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        if (pud_devmap(*pud)) {
                ptl = pud_lock(mm, pud);
                page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap);
                spin_unlock(ptl);
                if (page)
                        return page;
        }
        if (unlikely(pud_bad(*pud)))
                return no_page_table(vma, flags);

        return follow_pmd_mask(vma, address, pud, flags, ctx);
}

static struct page *follow_p4d_mask(struct vm_area_struct *vma,
                                    unsigned long address, pgd_t *pgdp,
                                    unsigned int flags,
                                    struct follow_page_context *ctx)
{
        p4d_t *p4d;
        struct page *page;

        p4d = p4d_offset(pgdp, address);
        if (p4d_none(*p4d))
                return no_page_table(vma, flags);
        BUILD_BUG_ON(p4d_huge(*p4d));
        if (unlikely(p4d_bad(*p4d)))
                return no_page_table(vma, flags);

        if (is_hugepd(__hugepd(p4d_val(*p4d)))) {
                page = follow_huge_pd(vma, address,
                                      __hugepd(p4d_val(*p4d)), flags,
                                      P4D_SHIFT);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        return follow_pud_mask(vma, address, p4d, flags, ctx);
}

/**
 * follow_page_mask - look up a page descriptor from a user-virtual address
 * @vma: vm_area_struct mapping @address
 * @address: virtual address to look up
 * @flags: flags modifying lookup behaviour
 * @ctx: contains dev_pagemap for %ZONE_DEVICE memory pinning and a
 *       pointer to output page_mask
 *
 * @flags can have FOLL_ flags set, defined in <linux/mm.h>
 *
 * When getting pages from ZONE_DEVICE memory, the @ctx->pgmap caches
 * the device's dev_pagemap metadata to avoid repeating expensive lookups.
 *
 * When getting an anonymous page and the caller has to trigger unsharing
 * of a shared anonymous page first, -EMLINK is returned. The caller should
 * trigger a fault with FAULT_FLAG_UNSHARE set. Note that unsharing is only
 * relevant with FOLL_PIN and !FOLL_WRITE.
 *
 * On output, the @ctx->page_mask is set according to the size of the page.
 *
 * Return: the mapped (struct page *), %NULL if no mapping exists, or
 * an error pointer if there is a mapping to something not represented
 * by a page descriptor (see also vm_normal_page()).
 */
static struct page *follow_page_mask(struct vm_area_struct *vma,
                              unsigned long address, unsigned int flags,
                              struct follow_page_context *ctx)
{
        pgd_t *pgd;
        struct page *page;
        struct mm_struct *mm = vma->vm_mm;

        ctx->page_mask = 0;

        /* make this handle hugepd */
        page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
        if (!IS_ERR(page)) {
                WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
                return page;
        }

        pgd = pgd_offset(mm, address);

        if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
                return no_page_table(vma, flags);

        if (pgd_huge(*pgd)) {
                page = follow_huge_pgd(mm, address, pgd, flags);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }
        if (is_hugepd(__hugepd(pgd_val(*pgd)))) {
                page = follow_huge_pd(vma, address,
                                      __hugepd(pgd_val(*pgd)), flags,
                                      PGDIR_SHIFT);
                if (page)
                        return page;
                return no_page_table(vma, flags);
        }

        return follow_p4d_mask(vma, address, pgd, flags, ctx);
}

struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
                         unsigned int foll_flags)
{
        struct follow_page_context ctx = { NULL };
        struct page *page;

        if (vma_is_secretmem(vma))
                return NULL;

        if (foll_flags & FOLL_PIN)
                return NULL;

        page = follow_page_mask(vma, address, foll_flags, &ctx);
        if (ctx.pgmap)
                put_dev_pagemap(ctx.pgmap);
        return page;
}

static int get_gate_page(struct mm_struct *mm, unsigned long address,
                unsigned int gup_flags, struct vm_area_struct **vma,
                struct page **page)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;
        int ret = -EFAULT;

        /* user gate pages are read-only */
        if (gup_flags & FOLL_WRITE)
                return -EFAULT;
        if (address > TASK_SIZE)
                pgd = pgd_offset_k(address);
        else
                pgd = pgd_offset_gate(mm, address);
        if (pgd_none(*pgd))
                return -EFAULT;
        p4d = p4d_offset(pgd, address);
        if (p4d_none(*p4d))
                return -EFAULT;
        pud = pud_offset(p4d, address);
        if (pud_none(*pud))
                return -EFAULT;
        pmd = pmd_offset(pud, address);
        if (!pmd_present(*pmd))
                return -EFAULT;
        VM_BUG_ON(pmd_trans_huge(*pmd));
        pte = pte_offset_map(pmd, address);
        if (pte_none(*pte))
                goto unmap;
        *vma = get_gate_vma(mm);
        if (!page)
                goto out;
        *page = vm_normal_page(*vma, address, *pte);
        if (!*page) {
                if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(*pte)))
                        goto unmap;
                *page = pte_page(*pte);
        }
        if (unlikely(!try_grab_page(*page, gup_flags))) {
                ret = -ENOMEM;
                goto unmap;
        }
out:
        ret = 0;
unmap:
        pte_unmap(pte);
        return ret;
}

/*
 * mmap_lock must be held on entry.  If @locked != NULL and *@flags
 * does not include FOLL_NOWAIT, the mmap_lock may be released.  If it
 * is, *@locked will be set to 0 and -EBUSY returned.
 */
static int faultin_page(struct vm_area_struct *vma,
                unsigned long address, unsigned int *flags, bool unshare,
                int *locked)
{
        unsigned int fault_flags = 0;
        vm_fault_t ret;

        if (*flags & FOLL_NOFAULT)
                return -EFAULT;
        if (*flags & FOLL_WRITE)
                fault_flags |= FAULT_FLAG_WRITE;
        if (*flags & FOLL_REMOTE)
                fault_flags |= FAULT_FLAG_REMOTE;
        if (locked)
                fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
        if (*flags & FOLL_NOWAIT)
                fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
        if (*flags & FOLL_TRIED) {
                /*
                 * Note: FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED
                 * can co-exist
                 */
                fault_flags |= FAULT_FLAG_TRIED;
        }
        if (unshare) {
                fault_flags |= FAULT_FLAG_UNSHARE;
                /* FAULT_FLAG_WRITE and FAULT_FLAG_UNSHARE are incompatible */
                VM_BUG_ON(fault_flags & FAULT_FLAG_WRITE);
        }

        ret = handle_mm_fault(vma, address, fault_flags, NULL);

        if (ret & VM_FAULT_COMPLETED) {
                /*
                 * With FAULT_FLAG_RETRY_NOWAIT we'll never release the
                 * mmap lock in the page fault handler. Sanity check this.
                 */
                WARN_ON_ONCE(fault_flags & FAULT_FLAG_RETRY_NOWAIT);
                if (locked)
                        *locked = 0;
                /*
                 * We should do the same as VM_FAULT_RETRY, but let's not
                 * return -EBUSY since that's not reflecting the reality of
                 * what has happened - we've just fully completed a page
                 * fault, with the mmap lock released.  Use -EAGAIN to show
                 * that we want to take the mmap lock _again_.
                 */
                return -EAGAIN;
        }

        if (ret & VM_FAULT_ERROR) {
                int err = vm_fault_to_errno(ret, *flags);

                if (err)
                        return err;
                BUG();
        }

        if (ret & VM_FAULT_RETRY) {
                if (locked && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
                        *locked = 0;
                return -EBUSY;
        }

        return 0;
}

static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
{
        vm_flags_t vm_flags = vma->vm_flags;
        int write = (gup_flags & FOLL_WRITE);
        int foreign = (gup_flags & FOLL_REMOTE);

        if (vm_flags & (VM_IO | VM_PFNMAP))
                return -EFAULT;

        if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
                return -EFAULT;

        if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
                return -EOPNOTSUPP;

        if (vma_is_secretmem(vma))
                return -EFAULT;

        if (write) {
                if (!(vm_flags & VM_WRITE)) {
                        if (!(gup_flags & FOLL_FORCE))
                                return -EFAULT;
                        /*
                         * We used to let the write,force case do COW in a
                         * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
                         * set a breakpoint in a read-only mapping of an
                         * executable, without corrupting the file (yet only
                         * when that file had been opened for writing!).
                         * Anon pages in shared mappings are surprising: now
                         * just reject it.
                         */
                        if (!is_cow_mapping(vm_flags))
                                return -EFAULT;
                }
        } else if (!(vm_flags & VM_READ)) {
                if (!(gup_flags & FOLL_FORCE))
                        return -EFAULT;
                /*
                 * Is there actually any vma we can reach here which does not
                 * have VM_MAYREAD set?
                 */
                if (!(vm_flags & VM_MAYREAD))
                        return -EFAULT;
        }
        /*
         * gups are always data accesses, not instruction
         * fetches, so execute=false here
         */
        if (!arch_vma_access_permitted(vma, write, false, foreign))
                return -EFAULT;
        return 0;
}

/**
 * __get_user_pages() - pin user pages in memory
 * @mm:         mm_struct of target mm
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying pin behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 * @locked:     whether we're still with the mmap_lock held
 *
 * Returns either number of pages pinned (which may be less than the
 * number requested), or an error. Details about the return value:
 *
 * -- If nr_pages is 0, returns 0.
 * -- If nr_pages is >0, but no pages were pinned, returns -errno.
 * -- If nr_pages is >0, and some pages were pinned, returns the number of
 *    pages pinned. Again, this may be less than nr_pages.
 * -- 0 return value is possible when the fault would need to be retried.
 *
 * The caller is responsible for releasing returned @pages, via put_page().
 *
 * @vmas are valid only as long as mmap_lock is held.
 *
 * Must be called with mmap_lock held.  It may be released.  See below.
 *
 * __get_user_pages walks a process's page tables and takes a reference to
 * each struct page that each user address corresponds to at a given
 * instant. That is, it takes the page that would be accessed if a user
 * thread accesses the given user virtual address at that instant.
 *
 * This does not guarantee that the page exists in the user mappings when
 * __get_user_pages returns, and there may even be a completely different
 * page there in some cases (eg. if mmapped pagecache has been invalidated
 * and subsequently re faulted). However it does guarantee that the page
 * won't be freed completely. And mostly callers simply care that the page
 * contains data that was valid *at some point in time*. Typically, an IO
 * or similar operation cannot guarantee anything stronger anyway because
 * locks can't be held over the syscall boundary.
 *
 * If @gup_flags & FOLL_WRITE == 0, the page must not be written to. If
 * the page is written to, set_page_dirty (or set_page_dirty_lock, as
 * appropriate) must be called after the page is finished with, and
 * before put_page is called.
 *
 * If @locked != NULL, *@locked will be set to 0 when mmap_lock is
 * released by an up_read().  That can happen if @gup_flags does not
 * have FOLL_NOWAIT.
 *
 * A caller using such a combination of @locked and @gup_flags
 * must therefore hold the mmap_lock for reading only, and recognize
 * when it's been released.  Otherwise, it must be held for either
 * reading or writing and will not be released.
 *
 * In most cases, get_user_pages or get_user_pages_fast should be used
 * instead of __get_user_pages. __get_user_pages should be used only if
 * you need some special @gup_flags.
 */
static long __get_user_pages(struct mm_struct *mm,
                unsigned long start, unsigned long nr_pages,
                unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas, int *locked)
{
        long ret = 0, i = 0;
        struct vm_area_struct *vma = NULL;
        struct follow_page_context ctx = { NULL };

        if (!nr_pages)
                return 0;

        start = untagged_addr(start);

        VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));

        do {
                struct page *page;
                unsigned int foll_flags = gup_flags;
                unsigned int page_increm;

                /* first iteration or cross vma bound */
                if (!vma || start >= vma->vm_end) {
                        vma = find_extend_vma(mm, start);
                        if (!vma && in_gate_area(mm, start)) {
                                ret = get_gate_page(mm, start & PAGE_MASK,
                                                gup_flags, &vma,
                                                pages ? &pages[i] : NULL);
                                if (ret)
                                        goto out;
                                ctx.page_mask = 0;
                                goto next_page;
                        }

                        if (!vma) {
                                ret = -EFAULT;
                                goto out;
                        }
                        ret = check_vma_flags(vma, gup_flags);
                        if (ret)
                                goto out;

                        if (is_vm_hugetlb_page(vma)) {
                                i = follow_hugetlb_page(mm, vma, pages, vmas,
                                                &start, &nr_pages, i,
                                                gup_flags, locked);
                                if (locked && *locked == 0) {
                                        /*
                                         * We've got a VM_FAULT_RETRY
                                         * and we've lost mmap_lock.
                                         * We must stop here.
                                         */
                                        BUG_ON(gup_flags & FOLL_NOWAIT);
                                        goto out;
                                }
                                continue;
                        }
                }
retry:
                /*
                 * If we have a pending SIGKILL, don't keep faulting pages and
                 * potentially allocating memory.
                 */
                if (fatal_signal_pending(current)) {
                        ret = -EINTR;
                        goto out;
                }
                cond_resched();

                page = follow_page_mask(vma, start, foll_flags, &ctx);
                if (!page || PTR_ERR(page) == -EMLINK) {
                        ret = faultin_page(vma, start, &foll_flags,
                                           PTR_ERR(page) == -EMLINK, locked);
                        switch (ret) {
                        case 0:
                                goto retry;
                        case -EBUSY:
                        case -EAGAIN:
                                ret = 0;
                                fallthrough;
                        case -EFAULT:
                        case -ENOMEM:
                        case -EHWPOISON:
                                goto out;
                        }
                        BUG();
                } else if (PTR_ERR(page) == -EEXIST) {
                        /*
                         * Proper page table entry exists, but no corresponding
                         * struct page. If the caller expects **pages to be
                         * filled in, bail out now, because that can't be done
                         * for this page.
                         */
                        if (pages) {
                                ret = PTR_ERR(page);
                                goto out;
                        }

                        goto next_page;
                } else if (IS_ERR(page)) {
                        ret = PTR_ERR(page);
                        goto out;
                }
                if (pages) {
                        pages[i] = page;
                        flush_anon_page(vma, page, start);
                        flush_dcache_page(page);
                        ctx.page_mask = 0;
                }
next_page:
                if (vmas) {
                        vmas[i] = vma;
                        ctx.page_mask = 0;
                }
                page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
                if (page_increm > nr_pages)
                        page_increm = nr_pages;
                i += page_increm;
                start += page_increm * PAGE_SIZE;
                nr_pages -= page_increm;
        } while (nr_pages);
out:
        if (ctx.pgmap)
                put_dev_pagemap(ctx.pgmap);
        return i ? i : ret;
}

static bool vma_permits_fault(struct vm_area_struct *vma,
                              unsigned int fault_flags)
{
        bool write   = !!(fault_flags & FAULT_FLAG_WRITE);
        bool foreign = !!(fault_flags & FAULT_FLAG_REMOTE);
        vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;

        if (!(vm_flags & vma->vm_flags))
                return false;

        /*
         * The architecture might have a hardware protection
         * mechanism other than read/write that can deny access.
         *
         * gup always represents data access, not instruction
         * fetches, so execute=false here:
         */
        if (!arch_vma_access_permitted(vma, write, false, foreign))
                return false;

        return true;
}

/**
 * fixup_user_fault() - manually resolve a user page fault
 * @mm:         mm_struct of target mm
 * @address:    user address
 * @fault_flags:flags to pass down to handle_mm_fault()
 * @unlocked:   did we unlock the mmap_lock while retrying, maybe NULL if caller
 *              does not allow retry. If NULL, the caller must guarantee
 *              that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY.
 *
 * This is meant to be called in the specific scenario where for locking reasons
 * we try to access user memory in atomic context (within a pagefault_disable()
 * section), this returns -EFAULT, and we want to resolve the user fault before
 * trying again.
 *
 * Typically this is meant to be used by the futex code.
 *
 * The main difference with get_user_pages() is that this function will
 * unconditionally call handle_mm_fault() which will in turn perform all the
 * necessary SW fixup of the dirty and young bits in the PTE, while
 * get_user_pages() only guarantees to update these in the struct page.
 *
 * This is important for some architectures where those bits also gate the
 * access permission to the page because they are maintained in software.  On
 * such architectures, gup() will not be enough to make a subsequent access
 * succeed.
 *
 * This function will not return with an unlocked mmap_lock. So it has not the
 * same semantics wrt the @mm->mmap_lock as does filemap_fault().
 */
int fixup_user_fault(struct mm_struct *mm,
                     unsigned long address, unsigned int fault_flags,
                     bool *unlocked)
{
        struct vm_area_struct *vma;
        vm_fault_t ret;

        address = untagged_addr(address);

        if (unlocked)
                fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;

retry:
        vma = find_extend_vma(mm, address);
        if (!vma || address < vma->vm_start)
                return -EFAULT;

        if (!vma_permits_fault(vma, fault_flags))
                return -EFAULT;

        if ((fault_flags & FAULT_FLAG_KILLABLE) &&
            fatal_signal_pending(current))
                return -EINTR;

        ret = handle_mm_fault(vma, address, fault_flags, NULL);

        if (ret & VM_FAULT_COMPLETED) {
                /*
                 * NOTE: it's a pity that we need to retake the lock here
                 * to pair with the unlock() in the callers. Ideally we
                 * could tell the callers so they do not need to unlock.
                 */
                mmap_read_lock(mm);
                *unlocked = true;
                return 0;
        }

        if (ret & VM_FAULT_ERROR) {
                int err = vm_fault_to_errno(ret, 0);

                if (err)
                        return err;
                BUG();
        }

        if (ret & VM_FAULT_RETRY) {
                mmap_read_lock(mm);
                *unlocked = true;
                fault_flags |= FAULT_FLAG_TRIED;
                goto retry;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(fixup_user_fault);

/*
 * Please note that this function, unlike __get_user_pages will not
 * return 0 for nr_pages > 0 without FOLL_NOWAIT
 */
static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
                                                unsigned long start,
                                                unsigned long nr_pages,
                                                struct page **pages,
                                                struct vm_area_struct **vmas,
                                                int *locked,
                                                unsigned int flags)
{
        long ret, pages_done;
        bool lock_dropped;

        if (locked) {
                /* if VM_FAULT_RETRY can be returned, vmas become invalid */
                BUG_ON(vmas);
                /* check caller initialized locked */
                BUG_ON(*locked != 1);
        }

        if (flags & FOLL_PIN)
                mm_set_has_pinned_flag(&mm->flags);

        /*
         * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
         * is to set FOLL_GET if the caller wants pages[] filled in (but has
         * carelessly failed to specify FOLL_GET), so keep doing that, but only
         * for FOLL_GET, not for the newer FOLL_PIN.
         *
         * FOLL_PIN always expects pages to be non-null, but no need to assert
         * that here, as any failures will be obvious enough.
         */
        if (pages && !(flags & FOLL_PIN))
                flags |= FOLL_GET;

        pages_done = 0;
        lock_dropped = false;
        for (;;) {
                ret = __get_user_pages(mm, start, nr_pages, flags, pages,
                                       vmas, locked);
                if (!locked)
                        /* VM_FAULT_RETRY couldn't trigger, bypass */
                        return ret;

                /* VM_FAULT_RETRY or VM_FAULT_COMPLETED cannot return errors */
                if (!*locked) {
                        BUG_ON(ret < 0);
                        BUG_ON(ret >= nr_pages);
                }

                if (ret > 0) {
                        nr_pages -= ret;
                        pages_done += ret;
                        if (!nr_pages)
                                break;
                }
                if (*locked) {
                        /*
                         * VM_FAULT_RETRY didn't trigger or it was a
                         * FOLL_NOWAIT.
                         */
                        if (!pages_done)
                                pages_done = ret;
                        break;
                }
                /*
                 * VM_FAULT_RETRY triggered, so seek to the faulting offset.
                 * For the prefault case (!pages) we only update counts.
                 */
                if (likely(pages))
                        pages += ret;
                start += ret << PAGE_SHIFT;
                lock_dropped = true;

retry:
                /*
                 * Repeat on the address that fired VM_FAULT_RETRY
                 * with both FAULT_FLAG_ALLOW_RETRY and
                 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
                 * by fatal signals, so we need to check it before we
                 * start trying again otherwise it can loop forever.
                 */

                if (fatal_signal_pending(current)) {
                        if (!pages_done)
                                pages_done = -EINTR;
                        break;
                }

                ret = mmap_read_lock_killable(mm);
                if (ret) {
                        BUG_ON(ret > 0);
                        if (!pages_done)
                                pages_done = ret;
                        break;
                }

                *locked = 1;
                ret = __get_user_pages(mm, start, 1, flags | FOLL_TRIED,
                                       pages, NULL, locked);
                if (!*locked) {
                        /* Continue to retry until we succeeded */
                        BUG_ON(ret != 0);
                        goto retry;
                }
                if (ret != 1) {
                        BUG_ON(ret > 1);
                        if (!pages_done)
                                pages_done = ret;
                        break;
                }
                nr_pages--;
                pages_done++;
                if (!nr_pages)
                        break;
                if (likely(pages))
                        pages++;
                start += PAGE_SIZE;
        }
        if (lock_dropped && *locked) {
                /*
                 * We must let the caller know we temporarily dropped the lock
                 * and so the critical section protected by it was lost.
                 */
                mmap_read_unlock(mm);
                *locked = 0;
        }
        return pages_done;
}

/**
 * populate_vma_page_range() -  populate a range of pages in the vma.
 * @vma:   target vma
 * @start: start address
 * @end:   end address
 * @locked: whether the mmap_lock is still held
 *
 * This takes care of mlocking the pages too if VM_LOCKED is set.
 *
 * Return either number of pages pinned in the vma, or a negative error
 * code on error.
 *
 * vma->vm_mm->mmap_lock must be held.
 *
 * If @locked is NULL, it may be held for read or write and will
 * be unperturbed.
 *
 * If @locked is non-NULL, it must held for read only and may be
 * released.  If it's released, *@locked will be set to 0.
 */
long populate_vma_page_range(struct vm_area_struct *vma,
                unsigned long start, unsigned long end, int *locked)
{
        struct mm_struct *mm = vma->vm_mm;
        unsigned long nr_pages = (end - start) / PAGE_SIZE;
        int gup_flags;
        long ret;

        VM_BUG_ON(!PAGE_ALIGNED(start));
        VM_BUG_ON(!PAGE_ALIGNED(end));
        VM_BUG_ON_VMA(start < vma->vm_start, vma);
        VM_BUG_ON_VMA(end   > vma->vm_end, vma);
        mmap_assert_locked(mm);

        /*
         * Rightly or wrongly, the VM_LOCKONFAULT case has never used
         * faultin_page() to break COW, so it has no work to do here.
         */
        if (vma->vm_flags & VM_LOCKONFAULT)
                return nr_pages;

        gup_flags = FOLL_TOUCH;
        /*
         * We want to touch writable mappings with a write fault in order
         * to break COW, except for shared mappings because these don't COW
         * and we would not want to dirty them for nothing.
         */
        if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
                gup_flags |= FOLL_WRITE;

        /*
         * We want mlock to succeed for regions that have any permissions
         * other than PROT_NONE.
         */
        if (vma_is_accessible(vma))
                gup_flags |= FOLL_FORCE;

        /*
         * We made sure addr is within a VMA, so the following will
         * not result in a stack expansion that recurses back here.
         */
        ret = __get_user_pages(mm, start, nr_pages, gup_flags,
                                NULL, NULL, locked);
        lru_add_drain();
        return ret;
}

/*
 * faultin_vma_page_range() - populate (prefault) page tables inside the
 *                            given VMA range readable/writable
 *
 * This takes care of mlocking the pages, too, if VM_LOCKED is set.
 *
 * @vma: target vma
 * @start: start address
 * @end: end address
 * @write: whether to prefault readable or writable
 * @locked: whether the mmap_lock is still held
 *
 * Returns either number of processed pages in the vma, or a negative error
 * code on error (see __get_user_pages()).
 *
 * vma->vm_mm->mmap_lock must be held. The range must be page-aligned and
 * covered by the VMA.
 *
 * If @locked is NULL, it may be held for read or write and will be unperturbed.
 *
 * If @locked is non-NULL, it must held for read only and may be released.  If
 * it's released, *@locked will be set to 0.
 */
long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
                            unsigned long end, bool write, int *locked)
{
        struct mm_struct *mm = vma->vm_mm;
        unsigned long nr_pages = (end - start) / PAGE_SIZE;
        int gup_flags;
        long ret;

        VM_BUG_ON(!PAGE_ALIGNED(start));
        VM_BUG_ON(!PAGE_ALIGNED(end));
        VM_BUG_ON_VMA(start < vma->vm_start, vma);
        VM_BUG_ON_VMA(end > vma->vm_end, vma);
        mmap_assert_locked(mm);

        /*
         * FOLL_TOUCH: Mark page accessed and thereby young; will also mark
         *             the page dirty with FOLL_WRITE -- which doesn't make a
         *             difference with !FOLL_FORCE, because the page is writable
         *             in the page table.
         * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit
         *                a poisoned page.
         * !FOLL_FORCE: Require proper access permissions.
         */
        gup_flags = FOLL_TOUCH | FOLL_HWPOISON;
        if (write)
                gup_flags |= FOLL_WRITE;

        /*
         * We want to report -EINVAL instead of -EFAULT for any permission
         * problems or incompatible mappings.
         */
        if (check_vma_flags(vma, gup_flags))
                return -EINVAL;

        ret = __get_user_pages(mm, start, nr_pages, gup_flags,
                                NULL, NULL, locked);
        lru_add_drain();
        return ret;
}

/*
 * __mm_populate - populate and/or mlock pages within a range of address space.
 *
 * This is used to implement mlock() and the MAP_POPULATE / MAP_LOCKED mmap
 * flags. VMAs must be already marked with the desired vm_flags, and
 * mmap_lock must not be held.
 */
int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
{
        struct mm_struct *mm = current->mm;
        unsigned long end, nstart, nend;
        struct vm_area_struct *vma = NULL;
        int locked = 0;
        long ret = 0;

        end = start + len;

        for (nstart = start; nstart < end; nstart = nend) {
                /*
                 * We want to fault in pages for [nstart; end) address range.
                 * Find first corresponding VMA.
                 */
                if (!locked) {
                        locked = 1;
                        mmap_read_lock(mm);
                        vma = find_vma_intersection(mm, nstart, end);
                } else if (nstart >= vma->vm_end)
                        vma = find_vma_intersection(mm, vma->vm_end, end);

                if (!vma)
                        break;
                /*
                 * Set [nstart; nend) to intersection of desired address
                 * range with the first VMA. Also, skip undesirable VMA types.
                 */
                nend = min(end, vma->vm_end);
                if (vma->vm_flags & (VM_IO | VM_PFNMAP))
                        continue;
                if (nstart < vma->vm_start)
                        nstart = vma->vm_start;
                /*
                 * Now fault in a range of pages. populate_vma_page_range()
                 * double checks the vma flags, so that it won't mlock pages
                 * if the vma was already munlocked.
                 */
                ret = populate_vma_page_range(vma, nstart, nend, &locked);
                if (ret < 0) {
                        if (ignore_errors) {
                                ret = 0;
                                continue;       /* continue at next VMA */
                        }
                        break;
                }
                nend = nstart + ret * PAGE_SIZE;
                ret = 0;
        }
        if (locked)
                mmap_read_unlock(mm);
        return ret;     /* 0 or negative error code */
}
#else /* CONFIG_MMU */
static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
                unsigned long nr_pages, struct page **pages,
                struct vm_area_struct **vmas, int *locked,
                unsigned int foll_flags)
{
        struct vm_area_struct *vma;
        unsigned long vm_flags;
        long i;

        /* calculate required read or write permissions.
         * If FOLL_FORCE is set, we only require the "MAY" flags.
         */
        vm_flags  = (foll_flags & FOLL_WRITE) ?
                        (VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
        vm_flags &= (foll_flags & FOLL_FORCE) ?
                        (VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);

        for (i = 0; i < nr_pages; i++) {
                vma = find_vma(mm, start);
                if (!vma)
                        goto finish_or_fault;

                /* protect what we can, including chardevs */
                if ((vma->vm_flags & (VM_IO | VM_PFNMAP)) ||
                    !(vm_flags & vma->vm_flags))
                        goto finish_or_fault;

                if (pages) {
                        pages[i] = virt_to_page((void *)start);
                        if (pages[i])
                                get_page(pages[i]);
                }
                if (vmas)
                        vmas[i] = vma;
                start = (start + PAGE_SIZE) & PAGE_MASK;
        }

        return i;

finish_or_fault:
        return i ? : -EFAULT;
}
#endif /* !CONFIG_MMU */

/**
 * fault_in_writeable - fault in userspace address range for writing
 * @uaddr: start of address range
 * @size: size of address range
 *
 * Returns the number of bytes not faulted in (like copy_to_user() and
 * copy_from_user()).
 */
size_t fault_in_writeable(char __user *uaddr, size_t size)
{
        char __user *start = uaddr, *end;

        if (unlikely(size == 0))
                return 0;
        if (!user_write_access_begin(uaddr, size))
                return size;
        if (!PAGE_ALIGNED(uaddr)) {
                unsafe_put_user(0, uaddr, out);
                uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
        }
        end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
        if (unlikely(end < start))
                end = NULL;
        while (uaddr != end) {
                unsafe_put_user(0, uaddr, out);
                uaddr += PAGE_SIZE;
        }

out:
        user_write_access_end();
        if (size > uaddr - start)
                return size - (uaddr - start);
        return 0;
}
EXPORT_SYMBOL(fault_in_writeable);

/**
 * fault_in_subpage_writeable - fault in an address range for writing
 * @uaddr: start of address range
 * @size: size of address range
 *
 * Fault in a user address range for writing while checking for permissions at
 * sub-page granularity (e.g. arm64 MTE). This function should be used when
 * the caller cannot guarantee forward progress of a copy_to_user() loop.
 *
 * Returns the number of bytes not faulted in (like copy_to_user() and
 * copy_from_user()).
 */
size_t fault_in_subpage_writeable(char __user *uaddr, size_t size)
{
        size_t faulted_in;

        /*
         * Attempt faulting in at page granularity first for page table
         * permission checking. The arch-specific probe_subpage_writeable()
         * functions may not check for this.
         */
        faulted_in = size - fault_in_writeable(uaddr, size);
        if (faulted_in)
                faulted_in -= probe_subpage_writeable(uaddr, faulted_in);

        return size - faulted_in;
}
EXPORT_SYMBOL(fault_in_subpage_writeable);

/*
 * fault_in_safe_writeable - fault in an address range for writing
 * @uaddr: start of address range
 * @size: length of address range
 *
 * Faults in an address range for writing.  This is primarily useful when we
 * already know that some or all of the pages in the address range aren't in
 * memory.
 *
 * Unlike fault_in_writeable(), this function is non-destructive.
 *
 * Note that we don't pin or otherwise hold the pages referenced that we fault
 * in.  There's no guarantee that they'll stay in memory for any duration of
 * time.
 *
 * Returns the number of bytes not faulted in, like copy_to_user() and
 * copy_from_user().
 */
size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
{
        unsigned long start = (unsigned long)uaddr, end;
        struct mm_struct *mm = current->mm;
        bool unlocked = false;

        if (unlikely(size == 0))
                return 0;
        end = PAGE_ALIGN(start + size);
        if (end < start)
                end = 0;

        mmap_read_lock(mm);
        do {
                if (fixup_user_fault(mm, start, FAULT_FLAG_WRITE, &unlocked))
                        break;
                start = (start + PAGE_SIZE) & PAGE_MASK;
        } while (start != end);
        mmap_read_unlock(mm);

        if (size > (unsigned long)uaddr - start)
                return size - ((unsigned long)uaddr - start);
        return 0;
}
EXPORT_SYMBOL(fault_in_safe_writeable);

/**
 * fault_in_readable - fault in userspace address range for reading
 * @uaddr: start of user address range
 * @size: size of user address range
 *
 * Returns the number of bytes not faulted in (like copy_to_user() and
 * copy_from_user()).
 */
size_t fault_in_readable(const char __user *uaddr, size_t size)
{
        const char __user *start = uaddr, *end;
        volatile char c;

        if (unlikely(size == 0))
                return 0;
        if (!user_read_access_begin(uaddr, size))
                return size;
        if (!PAGE_ALIGNED(uaddr)) {
                unsafe_get_user(c, uaddr, out);
                uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr);
        }
        end = (const char __user *)PAGE_ALIGN((unsigned long)start + size);
        if (unlikely(end < start))
                end = NULL;
        while (uaddr != end) {
                unsafe_get_user(c, uaddr, out);
                uaddr += PAGE_SIZE;
        }

out:
        user_read_access_end();
        (void)c;
        if (size > uaddr - start)
                return size - (uaddr - start);
        return 0;
}
EXPORT_SYMBOL(fault_in_readable);

/**
 * get_dump_page() - pin user page in memory while writing it to core dump
 * @addr: user address
 *
 * Returns struct page pointer of user page pinned for dump,
 * to be freed afterwards by put_page().
 *
 * Returns NULL on any kind of failure - a hole must then be inserted into
 * the corefile, to preserve alignment with its headers; and also returns
 * NULL wherever the ZERO_PAGE, or an anonymous pte_none, has been found -
 * allowing a hole to be left in the corefile to save disk space.
 *
 * Called without mmap_lock (takes and releases the mmap_lock by itself).
 */
#ifdef CONFIG_ELF_CORE
struct page *get_dump_page(unsigned long addr)
{
        struct mm_struct *mm = current->mm;
        struct page *page;
        int locked = 1;
        int ret;

        if (mmap_read_lock_killable(mm))
                return NULL;
        ret = __get_user_pages_locked(mm, addr, 1, &page, NULL, &locked,
                                      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
        if (locked)
                mmap_read_unlock(mm);
        return (ret == 1) ? page : NULL;
}
#endif /* CONFIG_ELF_CORE */

#ifdef CONFIG_MIGRATION
/*
 * Returns the number of collected pages. Return value is always >= 0.
 */
static unsigned long collect_longterm_unpinnable_pages(
                                        struct list_head *movable_page_list,
                                        unsigned long nr_pages,
                                        struct page **pages)
{
        unsigned long i, collected = 0;
        struct folio *prev_folio = NULL;
        bool drain_allow = true;

        for (i = 0; i < nr_pages; i++) {
                struct folio *folio = page_folio(pages[i]);

                if (folio == prev_folio)
                        continue;
                prev_folio = folio;

                if (folio_is_longterm_pinnable(folio))
                        continue;

                collected++;

                if (folio_is_device_coherent(folio))
                        continue;

                if (folio_test_hugetlb(folio)) {
                        isolate_hugetlb(&folio->page, movable_page_list);
                        continue;
                }

                if (!folio_test_lru(folio) && drain_allow) {
                        lru_add_drain_all();
                        drain_allow = false;
                }

                if (!folio_isolate_lru(folio))
                        continue;

                list_add_tail(&folio->lru, movable_page_list);
                node_stat_mod_folio(folio,
                                    NR_ISOLATED_ANON + folio_is_file_lru(folio),
                                    folio_nr_pages(folio));
        }

        return collected;
}

/*
 * Unpins all pages and migrates device coherent pages and movable_page_list.
 * Returns -EAGAIN if all pages were successfully migrated or -errno for failure
 * (or partial success).
 */
static int migrate_longterm_unpinnable_pages(
                                        struct list_head *movable_page_list,
                                        unsigned long nr_pages,
                                        struct page **pages)
{
        int ret;
        unsigned long i;

        for (i = 0; i < nr_pages; i++) {
                struct folio *folio = page_folio(pages[i]);

                if (folio_is_device_coherent(folio)) {
                        /*
                         * Migration will fail if the page is pinned, so convert
                         * the pin on the source page to a normal reference.
                         */
                        pages[i] = NULL;
                        folio_get(folio);
                        gup_put_folio(folio, 1, FOLL_PIN);

                        if (migrate_device_coherent_page(&folio->page)) {
                                ret = -EBUSY;
                                goto err;
                        }

                        continue;
                }

                /*
                 * We can't migrate pages with unexpected references, so drop
                 * the reference obtained by __get_user_pages_locked().
                 * Migrating pages have been added to movable_page_list after
                 * calling folio_isolate_lru() which takes a reference so the
                 * page won't be freed if it's migrating.
                 */
                unpin_user_page(pages[i]);
                pages[i] = NULL;
        }

        if (!list_empty(movable_page_list)) {
                struct migration_target_control mtc = {
                        .nid = NUMA_NO_NODE,
                        .gfp_mask = GFP_USER | __GFP_NOWARN,
                };

                if (migrate_pages(movable_page_list, alloc_migration_target,
                                  NULL, (unsigned long)&mtc, MIGRATE_SYNC,
                                  MR_LONGTERM_PIN, NULL)) {
                        ret = -ENOMEM;
                        goto err;
                }
        }

        putback_movable_pages(movable_page_list);

        return -EAGAIN;

err:
        for (i = 0; i < nr_pages; i++)
                if (pages[i])
                        unpin_user_page(pages[i]);
        putback_movable_pages(movable_page_list);

        return ret;
}

/*
 * Check whether all pages are *allowed* to be pinned. Rather confusingly, all
 * pages in the range are required to be pinned via FOLL_PIN, before calling
 * this routine.
 *
 * If any pages in the range are not allowed to be pinned, then this routine
 * will migrate those pages away, unpin all the pages in the range and return
 * -EAGAIN. The caller should re-pin the entire range with FOLL_PIN and then
 * call this routine again.
 *
 * If an error other than -EAGAIN occurs, this indicates a migration failure.
 * The caller should give up, and propagate the error back up the call stack.
 *
 * If everything is OK and all pages in the range are allowed to be pinned, then
 * this routine leaves all pages pinned and returns zero for success.
 */
static long check_and_migrate_movable_pages(unsigned long nr_pages,
                                            struct page **pages)
{
        unsigned long collected;
        LIST_HEAD(movable_page_list);

        collected = collect_longterm_unpinnable_pages(&movable_page_list,
                                                nr_pages, pages);
        if (!collected)
                return 0;

        return migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages,
                                                pages);
}
#else
static long check_and_migrate_movable_pages(unsigned long nr_pages,
                                            struct page **pages)
{
        return 0;
}
#endif /* CONFIG_MIGRATION */

/*
 * __gup_longterm_locked() is a wrapper for __get_user_pages_locked which
 * allows us to process the FOLL_LONGTERM flag.
 */
static long __gup_longterm_locked(struct mm_struct *mm,
                                  unsigned long start,
                                  unsigned long nr_pages,
                                  struct page **pages,
                                  struct vm_area_struct **vmas,
                                  unsigned int gup_flags)
{
        unsigned int flags;
        long rc, nr_pinned_pages;

        if (!(gup_flags & FOLL_LONGTERM))
                return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
                                               NULL, gup_flags);

        /*
         * If we get to this point then FOLL_LONGTERM is set, and FOLL_LONGTERM
         * implies FOLL_PIN (although the reverse is not true). Therefore it is
         * correct to unconditionally call check_and_migrate_movable_pages()
         * which assumes pages have been pinned via FOLL_PIN.
         *
         * Enforce the above reasoning by asserting that FOLL_PIN is set.
         */
        if (WARN_ON(!(gup_flags & FOLL_PIN)))
                return -EINVAL;
        flags = memalloc_pin_save();
        do {
                nr_pinned_pages = __get_user_pages_locked(mm, start, nr_pages,
                                                          pages, vmas, NULL,
                                                          gup_flags);
                if (nr_pinned_pages <= 0) {
                        rc = nr_pinned_pages;
                        break;
                }
                rc = check_and_migrate_movable_pages(nr_pinned_pages, pages);
        } while (rc == -EAGAIN);
        memalloc_pin_restore(flags);

        return rc ? rc : nr_pinned_pages;
}

static bool is_valid_gup_flags(unsigned int gup_flags)
{
        /*
         * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
         * never directly by the caller, so enforce that with an assertion:
         */
        if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
                return false;
        /*
         * FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying
         * that is, FOLL_LONGTERM is a specific case, more restrictive case of
         * FOLL_PIN.
         */
        if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
                return false;

        return true;
}

#ifdef CONFIG_MMU
static long __get_user_pages_remote(struct mm_struct *mm,
                                    unsigned long start, unsigned long nr_pages,
                                    unsigned int gup_flags, struct page **pages,
                                    struct vm_area_struct **vmas, int *locked)
{
        /*
         * Parts of FOLL_LONGTERM behavior are incompatible with
         * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
         * vmas. However, this only comes up if locked is set, and there are
         * callers that do request FOLL_LONGTERM, but do not set locked. So,
         * allow what we can.
         */
        if (gup_flags & FOLL_LONGTERM) {
                if (WARN_ON_ONCE(locked))
                        return -EINVAL;
                /*
                 * This will check the vmas (even if our vmas arg is NULL)
                 * and return -ENOTSUPP if DAX isn't allowed in this case:
                 */
                return __gup_longterm_locked(mm, start, nr_pages, pages,
                                             vmas, gup_flags | FOLL_TOUCH |
                                             FOLL_REMOTE);
        }

        return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
                                       locked,
                                       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
}

/**
 * get_user_pages_remote() - pin user pages in memory
 * @mm:         mm_struct of target mm
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying lookup behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 * @locked:     pointer to lock flag indicating whether lock is held and
 *              subsequently whether VM_FAULT_RETRY functionality can be
 *              utilised. Lock must initially be held.
 *
 * Returns either number of pages pinned (which may be less than the
 * number requested), or an error. Details about the return value:
 *
 * -- If nr_pages is 0, returns 0.
 * -- If nr_pages is >0, but no pages were pinned, returns -errno.
 * -- If nr_pages is >0, and some pages were pinned, returns the number of
 *    pages pinned. Again, this may be less than nr_pages.
 *
 * The caller is responsible for releasing returned @pages, via put_page().
 *
 * @vmas are valid only as long as mmap_lock is held.
 *
 * Must be called with mmap_lock held for read or write.
 *
 * get_user_pages_remote walks a process's page tables and takes a reference
 * to each struct page that each user address corresponds to at a given
 * instant. That is, it takes the page that would be accessed if a user
 * thread accesses the given user virtual address at that instant.
 *
 * This does not guarantee that the page exists in the user mappings when
 * get_user_pages_remote returns, and there may even be a completely different
 * page there in some cases (eg. if mmapped pagecache has been invalidated
 * and subsequently re faulted). However it does guarantee that the page
 * won't be freed completely. And mostly callers simply care that the page
 * contains data that was valid *at some point in time*. Typically, an IO
 * or similar operation cannot guarantee anything stronger anyway because
 * locks can't be held over the syscall boundary.
 *
 * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page
 * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
 * be called after the page is finished with, and before put_page is called.
 *
 * get_user_pages_remote is typically used for fewer-copy IO operations,
 * to get a handle on the memory by some means other than accesses
 * via the user virtual addresses. The pages may be submitted for
 * DMA to devices or accessed via their kernel linear mapping (via the
 * kmap APIs). Care should be taken to use the correct cache flushing APIs.
 *
 * See also get_user_pages_fast, for performance critical applications.
 *
 * get_user_pages_remote should be phased out in favor of
 * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
 * should use get_user_pages_remote because it cannot pass
 * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
 */
long get_user_pages_remote(struct mm_struct *mm,
                unsigned long start, unsigned long nr_pages,
                unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas, int *locked)
{
        if (!is_valid_gup_flags(gup_flags))
                return -EINVAL;

        return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
                                       pages, vmas, locked);
}
EXPORT_SYMBOL(get_user_pages_remote);

#else /* CONFIG_MMU */
long get_user_pages_remote(struct mm_struct *mm,
                           unsigned long start, unsigned long nr_pages,
                           unsigned int gup_flags, struct page **pages,
                           struct vm_area_struct **vmas, int *locked)
{
        return 0;
}

static long __get_user_pages_remote(struct mm_struct *mm,
                                    unsigned long start, unsigned long nr_pages,
                                    unsigned int gup_flags, struct page **pages,
                                    struct vm_area_struct **vmas, int *locked)
{
        return 0;
}
#endif /* !CONFIG_MMU */

/**
 * get_user_pages() - pin user pages in memory
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying lookup behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 *
 * This is the same as get_user_pages_remote(), just with a less-flexible
 * calling convention where we assume that the mm being operated on belongs to
 * the current task, and doesn't allow passing of a locked parameter.  We also
 * obviously don't pass FOLL_REMOTE in here.
 */
long get_user_pages(unsigned long start, unsigned long nr_pages,
                unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas)
{
        if (!is_valid_gup_flags(gup_flags))
                return -EINVAL;

        return __gup_longterm_locked(current->mm, start, nr_pages,
                                     pages, vmas, gup_flags | FOLL_TOUCH);
}
EXPORT_SYMBOL(get_user_pages);

/*
 * get_user_pages_unlocked() is suitable to replace the form:
 *
 *      mmap_read_lock(mm);
 *      get_user_pages(mm, ..., pages, NULL);
 *      mmap_read_unlock(mm);
 *
 *  with:
 *
 *      get_user_pages_unlocked(mm, ..., pages);
 *
 * It is functionally equivalent to get_user_pages_fast so
 * get_user_pages_fast should be used instead if specific gup_flags
 * (e.g. FOLL_FORCE) are not required.
 */
long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
                             struct page **pages, unsigned int gup_flags)
{
        struct mm_struct *mm = current->mm;
        int locked = 1;
        long ret;

        /*
         * FIXME: Current FOLL_LONGTERM behavior is incompatible with
         * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
         * vmas.  As there are no users of this flag in this call we simply
         * disallow this option for now.
         */
        if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
                return -EINVAL;

        mmap_read_lock(mm);
        ret = __get_user_pages_locked(mm, start, nr_pages, pages, NULL,
                                      &locked, gup_flags | FOLL_TOUCH);
        if (locked)
                mmap_read_unlock(mm);
        return ret;
}
EXPORT_SYMBOL(get_user_pages_unlocked);

/*
 * Fast GUP
 *
 * get_user_pages_fast attempts to pin user pages by walking the page
 * tables directly and avoids taking locks. Thus the walker needs to be
 * protected from page table pages being freed from under it, and should
 * block any THP splits.
 *
 * One way to achieve this is to have the walker disable interrupts, and
 * rely on IPIs from the TLB flushing code blocking before the page table
 * pages are freed. This is unsuitable for architectures that do not need
 * to broadcast an IPI when invalidating TLBs.
 *
 * Another way to achieve this is to batch up page table containing pages
 * belonging to more than one mm_user, then rcu_sched a callback to free those
 * pages. Disabling interrupts will allow the fast_gup walker to both block
 * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
 * (which is a relatively rare event). The code below adopts this strategy.
 *
 * Before activating this code, please be aware that the following assumptions
 * are currently made:
 *
 *  *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to
 *  free pages containing page tables or TLB flushing requires IPI broadcast.
 *
 *  *) ptes can be read atomically by the architecture.
 *
 *  *) access_ok is sufficient to validate userspace address ranges.
 *
 * The last two assumptions can be relaxed by the addition of helper functions.
 *
 * This code is based heavily on the PowerPC implementation by Nick Piggin.
 */
#ifdef CONFIG_HAVE_FAST_GUP

static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
                                            unsigned int flags,
                                            struct page **pages)
{
        while ((*nr) - nr_start) {
                struct page *page = pages[--(*nr)];

                ClearPageReferenced(page);
                if (flags & FOLL_PIN)
                        unpin_user_page(page);
                else
                        put_page(page);
        }
}

#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
/*
 * Fast-gup relies on pte change detection to avoid concurrent pgtable
 * operations.
 *
 * To pin the page, fast-gup needs to do below in order:
 * (1) pin the page (by prefetching pte), then (2) check pte not changed.
 *
 * For the rest of pgtable operations where pgtable updates can be racy
 * with fast-gup, we need to do (1) clear pte, then (2) check whether page
 * is pinned.
 *
 * Above will work for all pte-level operations, including THP split.
 *
 * For THP collapse, it's a bit more complicated because fast-gup may be
 * walking a pgtable page that is being freed (pte is still valid but pmd
 * can be cleared already).  To avoid race in such condition, we need to
 * also check pmd here to make sure pmd doesn't change (corresponds to
 * pmdp_collapse_flush() in the THP collapse code path).
 */
static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
                         unsigned long end, unsigned int flags,
                         struct page **pages, int *nr)
{
        struct dev_pagemap *pgmap = NULL;
        int nr_start = *nr, ret = 0;
        pte_t *ptep, *ptem;

        ptem = ptep = pte_offset_map(&pmd, addr);
        do {
                pte_t pte = ptep_get_lockless(ptep);
                struct page *page;
                struct folio *folio;

                if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
                        goto pte_unmap;

                if (!pte_access_permitted(pte, flags & FOLL_WRITE))
                        goto pte_unmap;

                if (pte_devmap(pte)) {
                        if (unlikely(flags & FOLL_LONGTERM))
                                goto pte_unmap;

                        pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
                        if (unlikely(!pgmap)) {
                                undo_dev_pagemap(nr, nr_start, flags, pages);
                                goto pte_unmap;
                        }
                } else if (pte_special(pte))
                        goto pte_unmap;

                VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
                page = pte_page(pte);

                folio = try_grab_folio(page, 1, flags);
                if (!folio)
                        goto pte_unmap;

                if (unlikely(page_is_secretmem(page))) {
                        gup_put_folio(folio, 1, flags);
                        goto pte_unmap;
                }

                if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
                    unlikely(pte_val(pte) != pte_val(*ptep))) {
                        gup_put_folio(folio, 1, flags);
                        goto pte_unmap;
                }

                if (!pte_write(pte) && gup_must_unshare(flags, page)) {
                        gup_put_folio(folio, 1, flags);
                        goto pte_unmap;
                }

                /*
                 * We need to make the page accessible if and only if we are
                 * going to access its content (the FOLL_PIN case).  Please
                 * see Documentation/core-api/pin_user_pages.rst for
                 * details.
                 */
                if (flags & FOLL_PIN) {
                        ret = arch_make_page_accessible(page);
                        if (ret) {
                                gup_put_folio(folio, 1, flags);
                                goto pte_unmap;
                        }
                }
                folio_set_referenced(folio);
                pages[*nr] = page;
                (*nr)++;
        } while (ptep++, addr += PAGE_SIZE, addr != end);

        ret = 1;

pte_unmap:
        if (pgmap)
                put_dev_pagemap(pgmap);
        pte_unmap(ptem);
        return ret;
}
#else

/*
 * If we can't determine whether or not a pte is special, then fail immediately
 * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
 * to be special.
 *
 * For a futex to be placed on a THP tail page, get_futex_key requires a
 * get_user_pages_fast_only implementation that can pin pages. Thus it's still
 * useful to have gup_huge_pmd even if we can't operate on ptes.
 */
static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
                         unsigned long end, unsigned int flags,
                         struct page **pages, int *nr)
{
        return 0;
}
#endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */

#if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
static int __gup_device_huge(unsigned long pfn, unsigned long addr,
                             unsigned long end, unsigned int flags,
                             struct page **pages, int *nr)
{
        int nr_start = *nr;
        struct dev_pagemap *pgmap = NULL;

        do {
                struct page *page = pfn_to_page(pfn);

                pgmap = get_dev_pagemap(pfn, pgmap);
                if (unlikely(!pgmap)) {
                        undo_dev_pagemap(nr, nr_start, flags, pages);
                        break;
                }
                SetPageReferenced(page);
                pages[*nr] = page;
                if (unlikely(!try_grab_page(page, flags))) {
                        undo_dev_pagemap(nr, nr_start, flags, pages);
                        break;
                }
                (*nr)++;
                pfn++;
        } while (addr += PAGE_SIZE, addr != end);

        put_dev_pagemap(pgmap);
        return addr == end;
}

static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                                 unsigned long end, unsigned int flags,
                                 struct page **pages, int *nr)
{
        unsigned long fault_pfn;
        int nr_start = *nr;

        fault_pfn = pmd_pfn(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
        if (!__gup_device_huge(fault_pfn, addr, end, flags, pages, nr))
                return 0;

        if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
                undo_dev_pagemap(nr, nr_start, flags, pages);
                return 0;
        }
        return 1;
}

static int __gup_device_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
                                 unsigned long end, unsigned int flags,
                                 struct page **pages, int *nr)
{
        unsigned long fault_pfn;
        int nr_start = *nr;

        fault_pfn = pud_pfn(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
        if (!__gup_device_huge(fault_pfn, addr, end, flags, pages, nr))
                return 0;

        if (unlikely(pud_val(orig) != pud_val(*pudp))) {
                undo_dev_pagemap(nr, nr_start, flags, pages);
                return 0;
        }
        return 1;
}
#else
static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                                 unsigned long end, unsigned int flags,
                                 struct page **pages, int *nr)
{
        BUILD_BUG();
        return 0;
}

static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
                                 unsigned long end, unsigned int flags,
                                 struct page **pages, int *nr)
{
        BUILD_BUG();
        return 0;
}
#endif

static int record_subpages(struct page *page, unsigned long addr,
                           unsigned long end, struct page **pages)
{
        int nr;

        for (nr = 0; addr != end; nr++, addr += PAGE_SIZE)
                pages[nr] = nth_page(page, nr);

        return nr;
}

#ifdef CONFIG_ARCH_HAS_HUGEPD
static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
                                      unsigned long sz)
{
        unsigned long __boundary = (addr + sz) & ~(sz-1);
        return (__boundary - 1 < end - 1) ? __boundary : end;
}

static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
                       unsigned long end, unsigned int flags,
                       struct page **pages, int *nr)
{
        unsigned long pte_end;
        struct page *page;
        struct folio *folio;
        pte_t pte;
        int refs;

        pte_end = (addr + sz) & ~(sz-1);
        if (pte_end < end)
                end = pte_end;

        pte = huge_ptep_get(ptep);

        if (!pte_access_permitted(pte, flags & FOLL_WRITE))
                return 0;

        /* hugepages are never "special" */
        VM_BUG_ON(!pfn_valid(pte_pfn(pte)));

        page = nth_page(pte_page(pte), (addr & (sz - 1)) >> PAGE_SHIFT);
        refs = record_subpages(page, addr, end, pages + *nr);

        folio = try_grab_folio(page, refs, flags);
        if (!folio)
                return 0;

        if (unlikely(pte_val(pte) != pte_val(*ptep))) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        if (!pte_write(pte) && gup_must_unshare(flags, &folio->page)) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        *nr += refs;
        folio_set_referenced(folio);
        return 1;
}

static int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
                unsigned int pdshift, unsigned long end, unsigned int flags,
                struct page **pages, int *nr)
{
        pte_t *ptep;
        unsigned long sz = 1UL << hugepd_shift(hugepd);
        unsigned long next;

        ptep = hugepte_offset(hugepd, addr, pdshift);
        do {
                next = hugepte_addr_end(addr, end, sz);
                if (!gup_hugepte(ptep, sz, addr, end, flags, pages, nr))
                        return 0;
        } while (ptep++, addr = next, addr != end);

        return 1;
}
#else
static inline int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
                unsigned int pdshift, unsigned long end, unsigned int flags,
                struct page **pages, int *nr)
{
        return 0;
}
#endif /* CONFIG_ARCH_HAS_HUGEPD */

static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                        unsigned long end, unsigned int flags,
                        struct page **pages, int *nr)
{
        struct page *page;
        struct folio *folio;
        int refs;

        if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
                return 0;

        if (pmd_devmap(orig)) {
                if (unlikely(flags & FOLL_LONGTERM))
                        return 0;
                return __gup_device_huge_pmd(orig, pmdp, addr, end, flags,
                                             pages, nr);
        }

        page = nth_page(pmd_page(orig), (addr & ~PMD_MASK) >> PAGE_SHIFT);
        refs = record_subpages(page, addr, end, pages + *nr);

        folio = try_grab_folio(page, refs, flags);
        if (!folio)
                return 0;

        if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        if (!pmd_write(orig) && gup_must_unshare(flags, &folio->page)) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        *nr += refs;
        folio_set_referenced(folio);
        return 1;
}

static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
                        unsigned long end, unsigned int flags,
                        struct page **pages, int *nr)
{
        struct page *page;
        struct folio *folio;
        int refs;

        if (!pud_access_permitted(orig, flags & FOLL_WRITE))
                return 0;

        if (pud_devmap(orig)) {
                if (unlikely(flags & FOLL_LONGTERM))
                        return 0;
                return __gup_device_huge_pud(orig, pudp, addr, end, flags,
                                             pages, nr);
        }

        page = nth_page(pud_page(orig), (addr & ~PUD_MASK) >> PAGE_SHIFT);
        refs = record_subpages(page, addr, end, pages + *nr);

        folio = try_grab_folio(page, refs, flags);
        if (!folio)
                return 0;

        if (unlikely(pud_val(orig) != pud_val(*pudp))) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        if (!pud_write(orig) && gup_must_unshare(flags, &folio->page)) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        *nr += refs;
        folio_set_referenced(folio);
        return 1;
}

static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
                        unsigned long end, unsigned int flags,
                        struct page **pages, int *nr)
{
        int refs;
        struct page *page;
        struct folio *folio;

        if (!pgd_access_permitted(orig, flags & FOLL_WRITE))
                return 0;

        BUILD_BUG_ON(pgd_devmap(orig));

        page = nth_page(pgd_page(orig), (addr & ~PGDIR_MASK) >> PAGE_SHIFT);
        refs = record_subpages(page, addr, end, pages + *nr);

        folio = try_grab_folio(page, refs, flags);
        if (!folio)
                return 0;

        if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

        *nr += refs;
        folio_set_referenced(folio);
        return 1;
}

static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned long end,
                unsigned int flags, struct page **pages, int *nr)
{
        unsigned long next;
        pmd_t *pmdp;

        pmdp = pmd_offset_lockless(pudp, pud, addr);
        do {
                pmd_t pmd = READ_ONCE(*pmdp);

                next = pmd_addr_end(addr, end);
                if (!pmd_present(pmd))
                        return 0;

                if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
                             pmd_devmap(pmd))) {
                        if (pmd_protnone(pmd) &&
                            !gup_can_follow_protnone(flags))
                                return 0;

                        if (!gup_huge_pmd(pmd, pmdp, addr, next, flags,
                                pages, nr))
                                return 0;

                } else if (unlikely(is_hugepd(__hugepd(pmd_val(pmd))))) {
                        /*
                         * architecture have different format for hugetlbfs
                         * pmd format and THP pmd format
                         */
                        if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
                                         PMD_SHIFT, next, flags, pages, nr))
                                return 0;
                } else if (!gup_pte_range(pmd, pmdp, addr, next, flags, pages, nr))
                        return 0;
        } while (pmdp++, addr = next, addr != end);

        return 1;
}

static int gup_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, unsigned long end,
                         unsigned int flags, struct page **pages, int *nr)
{
        unsigned long next;
        pud_t *pudp;

        pudp = pud_offset_lockless(p4dp, p4d, addr);
        do {
                pud_t pud = READ_ONCE(*pudp);

                next = pud_addr_end(addr, end);
                if (unlikely(!pud_present(pud)))
                        return 0;
                if (unlikely(pud_huge(pud))) {
                        if (!gup_huge_pud(pud, pudp, addr, next, flags,
                                          pages, nr))
                                return 0;
                } else if (unlikely(is_hugepd(__hugepd(pud_val(pud))))) {
                        if (!gup_huge_pd(__hugepd(pud_val(pud)), addr,
                                         PUD_SHIFT, next, flags, pages, nr))
                                return 0;
                } else if (!gup_pmd_range(pudp, pud, addr, next, flags, pages, nr))
                        return 0;
        } while (pudp++, addr = next, addr != end);

        return 1;
}

static int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr, unsigned long end,
                         unsigned int flags, struct page **pages, int *nr)
{
        unsigned long next;
        p4d_t *p4dp;

        p4dp = p4d_offset_lockless(pgdp, pgd, addr);
        do {
                p4d_t p4d = READ_ONCE(*p4dp);

                next = p4d_addr_end(addr, end);
                if (p4d_none(p4d))
                        return 0;
                BUILD_BUG_ON(p4d_huge(p4d));
                if (unlikely(is_hugepd(__hugepd(p4d_val(p4d))))) {
                        if (!gup_huge_pd(__hugepd(p4d_val(p4d)), addr,
                                         P4D_SHIFT, next, flags, pages, nr))
                                return 0;
                } else if (!gup_pud_range(p4dp, p4d, addr, next, flags, pages, nr))
                        return 0;
        } while (p4dp++, addr = next, addr != end);

        return 1;
}

static void gup_pgd_range(unsigned long addr, unsigned long end,
                unsigned int flags, struct page **pages, int *nr)
{
        unsigned long next;
        pgd_t *pgdp;

        pgdp = pgd_offset(current->mm, addr);
        do {
                pgd_t pgd = READ_ONCE(*pgdp);

                next = pgd_addr_end(addr, end);
                if (pgd_none(pgd))
                        return;
                if (unlikely(pgd_huge(pgd))) {
                        if (!gup_huge_pgd(pgd, pgdp, addr, next, flags,
                                          pages, nr))
                                return;
                } else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
                        if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
                                         PGDIR_SHIFT, next, flags, pages, nr))
                                return;
                } else if (!gup_p4d_range(pgdp, pgd, addr, next, flags, pages, nr))
                        return;
        } while (pgdp++, addr = next, addr != end);
}
#else
static inline void gup_pgd_range(unsigned long addr, unsigned long end,
                unsigned int flags, struct page **pages, int *nr)
{
}
#endif /* CONFIG_HAVE_FAST_GUP */

#ifndef gup_fast_permitted
/*
 * Check if it's allowed to use get_user_pages_fast_only() for the range, or
 * we need to fall back to the slow version:
 */
static bool gup_fast_permitted(unsigned long start, unsigned long end)
{
        return true;
}
#endif

static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
                                   unsigned int gup_flags, struct page **pages)
{
        int ret;

        /*
         * FIXME: FOLL_LONGTERM does not work with
         * get_user_pages_unlocked() (see comments in that function)
         */
        if (gup_flags & FOLL_LONGTERM) {
                mmap_read_lock(current->mm);
                ret = __gup_longterm_locked(current->mm,
                                            start, nr_pages,
                                            pages, NULL, gup_flags);
                mmap_read_unlock(current->mm);
        } else {
                ret = get_user_pages_unlocked(start, nr_pages,
                                              pages, gup_flags);
        }

        return ret;
}

static unsigned long lockless_pages_from_mm(unsigned long start,
                                            unsigned long end,
                                            unsigned int gup_flags,
                                            struct page **pages)
{
        unsigned long flags;
        int nr_pinned = 0;
        unsigned seq;

        if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
            !gup_fast_permitted(start, end))
                return 0;

        if (gup_flags & FOLL_PIN) {
                seq = raw_read_seqcount(&current->mm->write_protect_seq);
                if (seq & 1)
                        return 0;
        }

        /*
         * Disable interrupts. The nested form is used, in order to allow full,
         * general purpose use of this routine.
         *
         * With interrupts disabled, we block page table pages from being freed
         * from under us. See struct mmu_table_batch comments in
         * include/asm-generic/tlb.h for more details.
         *
         * We do not adopt an rcu_read_lock() here as we also want to block IPIs
         * that come from THPs splitting.
         */
        local_irq_save(flags);
        gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
        local_irq_restore(flags);

        /*
         * When pinning pages for DMA there could be a concurrent write protect
         * from fork() via copy_page_range(), in this case always fail fast GUP.
         */
        if (gup_flags & FOLL_PIN) {
                if (read_seqcount_retry(&current->mm->write_protect_seq, seq)) {
                        unpin_user_pages_lockless(pages, nr_pinned);
                        return 0;
                } else {
                        sanity_check_pinned_pages(pages, nr_pinned);
                }
        }
        return nr_pinned;
}

static int internal_get_user_pages_fast(unsigned long start,
                                        unsigned long nr_pages,
                                        unsigned int gup_flags,
                                        struct page **pages)
{
        unsigned long len, end;
        unsigned long nr_pinned;
        int ret;

        if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
                                       FOLL_FORCE | FOLL_PIN | FOLL_GET |
                                       FOLL_FAST_ONLY | FOLL_NOFAULT)))
                return -EINVAL;

        if (gup_flags & FOLL_PIN)
                mm_set_has_pinned_flag(&current->mm->flags);

        if (!(gup_flags & FOLL_FAST_ONLY))
                might_lock_read(&current->mm->mmap_lock);

        start = untagged_addr(start) & PAGE_MASK;
        len = nr_pages << PAGE_SHIFT;
        if (check_add_overflow(start, len, &end))
                return 0;
        if (unlikely(!access_ok((void __user *)start, len)))
                return -EFAULT;

        nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
        if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
                return nr_pinned;

        /* Slow path: try to get the remaining pages with get_user_pages */
        start += nr_pinned << PAGE_SHIFT;
        pages += nr_pinned;
        ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
                                      pages);
        if (ret < 0) {
                /*
                 * The caller has to unpin the pages we already pinned so
                 * returning -errno is not an option
                 */
                if (nr_pinned)
                        return nr_pinned;
                return ret;
        }
        return ret + nr_pinned;
}

/**
 * get_user_pages_fast_only() - pin user pages in memory
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying pin behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 *
 * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
 * the regular GUP.
 * Note a difference with get_user_pages_fast: this always returns the
 * number of pages pinned, 0 if no pages were pinned.
 *
 * If the architecture does not support this function, simply return with no
 * pages pinned.
 *
 * Careful, careful! COW breaking can go either way, so a non-write
 * access can get ambiguous page results. If you call this function without
 * 'write' set, you'd better be sure that you're ok with that ambiguity.
 */
int get_user_pages_fast_only(unsigned long start, int nr_pages,
                             unsigned int gup_flags, struct page **pages)
{
        int nr_pinned;
        /*
         * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
         * because gup fast is always a "pin with a +1 page refcount" request.
         *
         * FOLL_FAST_ONLY is required in order to match the API description of
         * this routine: no fall back to regular ("slow") GUP.
         */
        gup_flags |= FOLL_GET | FOLL_FAST_ONLY;

        nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
                                                 pages);

        /*
         * As specified in the API description above, this routine is not
         * allowed to return negative values. However, the common core
         * routine internal_get_user_pages_fast() *can* return -errno.
         * Therefore, correct for that here:
         */
        if (nr_pinned < 0)
                nr_pinned = 0;

        return nr_pinned;
}
EXPORT_SYMBOL_GPL(get_user_pages_fast_only);

/**
 * get_user_pages_fast() - pin user pages in memory
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying pin behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 *
 * Attempt to pin user pages in memory without taking mm->mmap_lock.
 * If not successful, it will fall back to taking the lock and
 * calling get_user_pages().
 *
 * Returns number of pages pinned. This may be fewer than the number requested.
 * If nr_pages is 0 or negative, returns 0. If no pages were pinned, returns
 * -errno.
 */
int get_user_pages_fast(unsigned long start, int nr_pages,
                        unsigned int gup_flags, struct page **pages)
{
        if (!is_valid_gup_flags(gup_flags))
                return -EINVAL;

        /*
         * The caller may or may not have explicitly set FOLL_GET; either way is
         * OK. However, internally (within mm/gup.c), gup fast variants must set
         * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
         * request.
         */
        gup_flags |= FOLL_GET;
        return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
}
EXPORT_SYMBOL_GPL(get_user_pages_fast);

/**
 * pin_user_pages_fast() - pin user pages in memory without taking locks
 *
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying pin behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 *
 * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
 * get_user_pages_fast() for documentation on the function arguments, because
 * the arguments here are identical.
 *
 * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
 * see Documentation/core-api/pin_user_pages.rst for further details.
 */
int pin_user_pages_fast(unsigned long start, int nr_pages,
                        unsigned int gup_flags, struct page **pages)
{
        /* FOLL_GET and FOLL_PIN are mutually exclusive. */
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;

        if (WARN_ON_ONCE(!pages))
                return -EINVAL;

        gup_flags |= FOLL_PIN;
        return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
}
EXPORT_SYMBOL_GPL(pin_user_pages_fast);

/*
 * This is the FOLL_PIN equivalent of get_user_pages_fast_only(). Behavior
 * is the same, except that this one sets FOLL_PIN instead of FOLL_GET.
 *
 * The API rules are the same, too: no negative values may be returned.
 */
int pin_user_pages_fast_only(unsigned long start, int nr_pages,
                             unsigned int gup_flags, struct page **pages)
{
        int nr_pinned;

        /*
         * FOLL_GET and FOLL_PIN are mutually exclusive. Note that the API
         * rules require returning 0, rather than -errno:
         */
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return 0;

        if (WARN_ON_ONCE(!pages))
                return 0;
        /*
         * FOLL_FAST_ONLY is required in order to match the API description of
         * this routine: no fall back to regular ("slow") GUP.
         */
        gup_flags |= (FOLL_PIN | FOLL_FAST_ONLY);
        nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
                                                 pages);
        /*
         * This routine is not allowed to return negative values. However,
         * internal_get_user_pages_fast() *can* return -errno. Therefore,
         * correct for that here:
         */
        if (nr_pinned < 0)
                nr_pinned = 0;

        return nr_pinned;
}
EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);

/**
 * pin_user_pages_remote() - pin pages of a remote process
 *
 * @mm:         mm_struct of target mm
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying lookup behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 * @locked:     pointer to lock flag indicating whether lock is held and
 *              subsequently whether VM_FAULT_RETRY functionality can be
 *              utilised. Lock must initially be held.
 *
 * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
 * get_user_pages_remote() for documentation on the function arguments, because
 * the arguments here are identical.
 *
 * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
 * see Documentation/core-api/pin_user_pages.rst for details.
 */
long pin_user_pages_remote(struct mm_struct *mm,
                           unsigned long start, unsigned long nr_pages,
                           unsigned int gup_flags, struct page **pages,
                           struct vm_area_struct **vmas, int *locked)
{
        /* FOLL_GET and FOLL_PIN are mutually exclusive. */
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;

        if (WARN_ON_ONCE(!pages))
                return -EINVAL;

        gup_flags |= FOLL_PIN;
        return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
                                       pages, vmas, locked);
}
EXPORT_SYMBOL(pin_user_pages_remote);

/**
 * pin_user_pages() - pin user pages in memory for use by other devices
 *
 * @start:      starting user address
 * @nr_pages:   number of pages from start to pin
 * @gup_flags:  flags modifying lookup behaviour
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long.
 * @vmas:       array of pointers to vmas corresponding to each page.
 *              Or NULL if the caller does not require them.
 *
 * Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
 * FOLL_PIN is set.
 *
 * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
 * see Documentation/core-api/pin_user_pages.rst for details.
 */
long pin_user_pages(unsigned long start, unsigned long nr_pages,
                    unsigned int gup_flags, struct page **pages,
                    struct vm_area_struct **vmas)
{
        /* FOLL_GET and FOLL_PIN are mutually exclusive. */
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;

        if (WARN_ON_ONCE(!pages))
                return -EINVAL;

        gup_flags |= FOLL_PIN;
        return __gup_longterm_locked(current->mm, start, nr_pages,
                                     pages, vmas, gup_flags);
}
EXPORT_SYMBOL(pin_user_pages);

/*
 * pin_user_pages_unlocked() is the FOLL_PIN variant of
 * get_user_pages_unlocked(). Behavior is the same, except that this one sets
 * FOLL_PIN and rejects FOLL_GET.
 */
long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
                             struct page **pages, unsigned int gup_flags)
{
        /* FOLL_GET and FOLL_PIN are mutually exclusive. */
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;

        if (WARN_ON_ONCE(!pages))
                return -EINVAL;

        gup_flags |= FOLL_PIN;
        return get_user_pages_unlocked(start, nr_pages, pages, gup_flags);
}
EXPORT_SYMBOL(pin_user_pages_unlocked);