mm/swap: swap_vma_readahead() do the pte_offset_map()

swap_vma_readahead() has been proceeding in an unconventional way, its
preliminary swap_ra_info() doing the pte_offset_map() and pte_unmap(),
then relying on that pte pointer even after the pte_unmap() - in its
CONFIG_64BIT case (I think !CONFIG_HIGHPTE was intended; whereas 32-bit
copied ptes to stack while they were mapped, but had to limit how many).

Though it would be difficult to construct a failing testcase, accessing
page table after pte_unmap() will become bad practice, even on 64-bit: an
rcu_read_unlock() in pte_unmap() will allow page table to be freed.

Move relevant definitions from include/linux/swap.h to mm/swap_state.c,
nothing else used them.  Delete the CONFIG_64BIT distinction and buffer,
delete all reference to ptes from swap_ra_info(), use pte_offset_map()
repeatedly in swap_vma_readahead(), breaking from the loop if it fails.

(Will the repeated "map" and "unmap" show up as a slowdown anywhere?  If
so, maybe modify __read_swap_cache_async() to do the pte_unmap() only when
it does not find the page already in the swapcache.)

Use ptep_get_lockless(), mainly for its READ_ONCE().  Correctly advance
the address passed down to each call of __read__swap_cache_async().

Link: https://lkml.kernel.org/r/b7c64ab3-9e44-aac0-d2b-c57de578af1c@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zack Rusin <zackr@vmware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index b5f6f29..ce7e82c 100644 (file)
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -337,25 +337,6 @@ struct swap_info_struct {
                                           */
 };
 
-#ifdef CONFIG_64BIT
-#define SWAP_RA_ORDER_CEILING  5
-#else
-/* Avoid stack overflow, because we need to save part of page table */
-#define SWAP_RA_ORDER_CEILING  3
-#define SWAP_RA_PTE_CACHE_SIZE (1 << SWAP_RA_ORDER_CEILING)
-#endif
-
-struct vma_swap_readahead {
-       unsigned short win;
-       unsigned short offset;
-       unsigned short nr_pte;
-#ifdef CONFIG_64BIT
-       pte_t *ptes;
-#else
-       pte_t ptes[SWAP_RA_PTE_CACHE_SIZE];
-#endif
-};
-
 static inline swp_entry_t folio_swap_entry(struct folio *folio)
 {
        swp_entry_t entry = { .val = page_private(&folio->page) };

diff --git a/mm/swap_state.c b/mm/swap_state.c
index ef32353..a33c60e 100644 (file)
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -709,6 +709,14 @@ void exit_swap_address_space(unsigned int type)
        swapper_spaces[type] = NULL;
 }
 
+#define SWAP_RA_ORDER_CEILING  5
+
+struct vma_swap_readahead {
+       unsigned short win;
+       unsigned short offset;
+       unsigned short nr_pte;
+};
+
 static void swap_ra_info(struct vm_fault *vmf,
                         struct vma_swap_readahead *ra_info)
 {
@@ -716,11 +724,7 @@ static void swap_ra_info(struct vm_fault *vmf,
        unsigned long ra_val;
        unsigned long faddr, pfn, fpfn, lpfn, rpfn;
        unsigned long start, end;
-       pte_t *pte, *orig_pte;
        unsigned int max_win, hits, prev_win, win;
-#ifndef CONFIG_64BIT
-       pte_t *tpte;
-#endif
 
        max_win = 1 << min_t(unsigned int, READ_ONCE(page_cluster),
                             SWAP_RA_ORDER_CEILING);
@@ -739,12 +743,9 @@ static void swap_ra_info(struct vm_fault *vmf,
                                               max_win, prev_win);
        atomic_long_set(&vma->swap_readahead_info,
                        SWAP_RA_VAL(faddr, win, 0));
-
        if (win == 1)
                return;
 
-       /* Copy the PTEs because the page table may be unmapped */
-       orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
        if (fpfn == pfn + 1) {
                lpfn = fpfn;
                rpfn = fpfn + win;
@@ -764,15 +765,6 @@ static void swap_ra_info(struct vm_fault *vmf,
 
        ra_info->nr_pte = end - start;
        ra_info->offset = fpfn - start;
-       pte -= ra_info->offset;
-#ifdef CONFIG_64BIT
-       ra_info->ptes = pte;
-#else
-       tpte = ra_info->ptes;
-       for (pfn = start; pfn != end; pfn++)
-               *tpte++ = *pte++;
-#endif
-       pte_unmap(orig_pte);
 }
 
 /**
@@ -796,7 +788,8 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
        struct swap_iocb *splug = NULL;
        struct vm_area_struct *vma = vmf->vma;
        struct page *page;
-       pte_t *pte, pentry;
+       pte_t *pte = NULL, pentry;
+       unsigned long addr;
        swp_entry_t entry;
        unsigned int i;
        bool page_allocated;
@@ -808,17 +801,25 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
        if (ra_info.win == 1)
                goto skip;
 
+       addr = vmf->address - (ra_info.offset * PAGE_SIZE);
+
        blk_start_plug(&plug);
-       for (i = 0, pte = ra_info.ptes; i < ra_info.nr_pte;
-            i++, pte++) {
-               pentry = *pte;
+       for (i = 0; i < ra_info.nr_pte; i++, addr += PAGE_SIZE) {
+               if (!pte++) {
+                       pte = pte_offset_map(vmf->pmd, addr);
+                       if (!pte)
+                               break;
+               }
+               pentry = ptep_get_lockless(pte);
                if (!is_swap_pte(pentry))
                        continue;
                entry = pte_to_swp_entry(pentry);
                if (unlikely(non_swap_entry(entry)))
                        continue;
+               pte_unmap(pte);
+               pte = NULL;
                page = __read_swap_cache_async(entry, gfp_mask, vma,
-                                              vmf->address, &page_allocated);
+                                              addr, &page_allocated);
                if (!page)
                        continue;
                if (page_allocated) {
@@ -830,6 +831,8 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
                }
                put_page(page);
        }
+       if (pte)
+               pte_unmap(pte);
        blk_finish_plug(&plug);
        swap_read_unplug(splug);
        lru_add_drain();