mm/gup: reliable R/O long-term pinning in COW mappings

We already support reliable R/O pinning of anonymous memory. However,
assume we end up pinning (R/O long-term) a pagecache page or the shared
zeropage inside a writable private ("COW") mapping. The next write access
will trigger a write-fault and replace the pinned page by an exclusive
anonymous page in the process page tables to break COW: the pinned page no
longer corresponds to the page mapped into the process' page table.

Now that FAULT_FLAG_UNSHARE can break COW on anything mapped into a
COW mapping, let's properly break COW first before R/O long-term
pinning something that's not an exclusive anon page inside a COW
mapping. FAULT_FLAG_UNSHARE will break COW and map an exclusive anon page
instead that can get pinned safely.

With this change, we can stop using FOLL_FORCE|FOLL_WRITE for reliable
R/O long-term pinning in COW mappings.

With this change, the new R/O long-term pinning tests for non-anonymous
memory succeed:
  # [RUN] R/O longterm GUP pin ... with shared zeropage
  ok 151 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP pin ... with memfd
  ok 152 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP pin ... with tmpfile
  ok 153 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP pin ... with huge zeropage
  ok 154 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB)
  ok 155 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB)
  ok 156 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP-fast pin ... with shared zeropage
  ok 157 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP-fast pin ... with memfd
  ok 158 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP-fast pin ... with tmpfile
  ok 159 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP-fast pin ... with huge zeropage
  ok 160 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB)
  ok 161 Longterm R/O pin is reliable
  # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB)
  ok 162 Longterm R/O pin is reliable

Note 1: We don't care about short-term R/O-pinning, because they have
snapshot semantics: they are not supposed to observe modifications that
happen after pinning.

As one example, assume we start direct I/O to read from a page and store
page content into a file: modifications to page content after starting
direct I/O are not guaranteed to end up in the file. So even if we'd pin
the shared zeropage, the end result would be as expected -- getting zeroes
stored to the file.

Note 2: For shared mappings we'll now always fallback to the slow path to
lookup the VMA when R/O long-term pining. While that's the necessary price
we have to pay right now, it's actually not that bad in practice: most
FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with
FOLL_FORCE because they tried dealing with COW mappings correctly ...

Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE,
such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd
populate exclusive anon pages that we can pin. There was a concern that
this could affect the memlock limit of existing setups.

For example, a VM running with VFIO could run into the memlock limit and
fail to run. However, we essentially had the same behavior already in
commit 17839856fd58 ("gup: document and work around "COW can break either
way" issue") which got merged into some enterprise distros, and there were
not any such complaints. So most probably, we're fine.

Link: https://lkml.kernel.org/r/20221116102659.70287-10-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 686879d..d8363ac 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3149,8 +3149,12 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
  * Must be called with the (sub)page that's actually referenced via the
  * page table entry, which might not necessarily be the head page for a
  * PTE-mapped THP.
+ *
+ * If the vma is NULL, we're coming from the GUP-fast path and might have
+ * to fallback to the slow path just to lookup the vma.
  */
-static inline bool gup_must_unshare(unsigned int flags, struct page *page)
+static inline bool gup_must_unshare(struct vm_area_struct *vma,
+                                   unsigned int flags, struct page *page)
 {
        /*
         * FOLL_WRITE is implicitly handled correctly as the page table entry
@@ -3163,8 +3167,25 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page)
         * Note: PageAnon(page) is stable until the page is actually getting
         * freed.
         */
-       if (!PageAnon(page))
-               return false;
+       if (!PageAnon(page)) {
+               /*
+                * We only care about R/O long-term pining: R/O short-term
+                * pinning does not have the semantics to observe successive
+                * changes through the process page tables.
+                */
+               if (!(flags & FOLL_LONGTERM))
+                       return false;
+
+               /* We really need the vma ... */
+               if (!vma)
+                       return true;
+
+               /*
+                * ... because we only care about writable private ("COW")
+                * mappings where we have to break COW early.
+                */
+               return is_cow_mapping(vma->vm_flags);
+       }
 
        /* Paired with a memory barrier in page_try_share_anon_rmap(). */
        if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))

diff --git a/mm/gup.c b/mm/gup.c
index 2500d00..39c84a2 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -603,7 +603,7 @@ retry:
                }
        }
 
-       if (!pte_write(pte) && gup_must_unshare(flags, page)) {
+       if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) {
                page = ERR_PTR(-EMLINK);
                goto out;
        }
@@ -2380,7 +2380,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
                        goto pte_unmap;
                }
 
-               if (!pte_write(pte) && gup_must_unshare(flags, page)) {
+               if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) {
                        gup_put_folio(folio, 1, flags);
                        goto pte_unmap;
                }
@@ -2566,7 +2566,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
                return 0;
        }
 
-       if (!pte_write(pte) && gup_must_unshare(flags, &folio->page)) {
+       if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }
@@ -2632,7 +2632,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
                return 0;
        }
 
-       if (!pmd_write(orig) && gup_must_unshare(flags, &folio->page)) {
+       if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }
@@ -2672,7 +2672,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
                return 0;
        }
 
-       if (!pud_write(orig) && gup_must_unshare(flags, &folio->page)) {
+       if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
                gup_put_folio(folio, refs, flags);
                return 0;
        }

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 5eb7027..86a3004 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1480,7 +1480,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
        if (pmd_protnone(*pmd) && !gup_can_follow_protnone(flags))
                return NULL;
 
-       if (!pmd_write(*pmd) && gup_must_unshare(flags, page))
+       if (!pmd_write(*pmd) && gup_must_unshare(vma, flags, page))
                return ERR_PTR(-EMLINK);
 
        VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3d381b2..9d97c9a 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6197,7 +6197,8 @@ static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
        }
 }
 
-static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
+static inline bool __follow_hugetlb_must_fault(struct vm_area_struct *vma,
+                                              unsigned int flags, pte_t *pte,
                                               bool *unshare)
 {
        pte_t pteval = huge_ptep_get(pte);
@@ -6209,7 +6210,7 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
                return false;
        if (flags & FOLL_WRITE)
                return true;
-       if (gup_must_unshare(flags, pte_page(pteval))) {
+       if (gup_must_unshare(vma, flags, pte_page(pteval))) {
                *unshare = true;
                return true;
        }
@@ -6338,7 +6339,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
                 * directly from any kind of swap entries.
                 */
                if (absent ||
-                   __follow_hugetlb_must_fault(flags, pte, &unshare)) {
+                   __follow_hugetlb_must_fault(vma, flags, pte, &unshare)) {
                        vm_fault_t ret;
                        unsigned int fault_flags = 0;