--- /dev/null
+From 5abfd71d936a8aefd9f9ccd299dea7a164a5d455 Mon Sep 17 00:00:00 2001
+From: Peter Xu <peterx@redhat.com>
+Date: Tue, 22 Mar 2022 14:42:15 -0700
+Subject: mm: don't skip swap entry even if zap_details specified
+
+From: Peter Xu <peterx@redhat.com>
+
+commit 5abfd71d936a8aefd9f9ccd299dea7a164a5d455 upstream.
+
+Patch series "mm: Rework zap ptes on swap entries", v5.
+
+Patch 1 should fix a long standing bug for zap_pte_range() on
+zap_details usage. The risk is we could have some swap entries skipped
+while we should have zapped them.
+
+Migration entries are not the major concern because file backed memory
+always zap in the pattern that "first time without page lock, then
+re-zap with page lock" hence the 2nd zap will always make sure all
+migration entries are already recovered.
+
+However there can be issues with real swap entries got skipped
+errornoously. There's a reproducer provided in commit message of patch
+1 for that.
+
+Patch 2-4 are cleanups that are based on patch 1. After the whole
+patchset applied, we should have a very clean view of zap_pte_range().
+
+Only patch 1 needs to be backported to stable if necessary.
+
+This patch (of 4):
+
+The "details" pointer shouldn't be the token to decide whether we should
+skip swap entries.
+
+For example, when the callers specified details->zap_mapping==NULL, it
+means the user wants to zap all the pages (including COWed pages), then
+we need to look into swap entries because there can be private COWed
+pages that was swapped out.
+
+Skipping some swap entries when details is non-NULL may lead to wrongly
+leaving some of the swap entries while we should have zapped them.
+
+A reproducer of the problem:
+
+===8<===
+ #define _GNU_SOURCE /* See feature_test_macros(7) */
+ #include <stdio.h>
+ #include <assert.h>
+ #include <unistd.h>
+ #include <sys/mman.h>
+ #include <sys/types.h>
+
+ int page_size;
+ int shmem_fd;
+ char *buffer;
+
+ void main(void)
+ {
+ int ret;
+ char val;
+
+ page_size = getpagesize();
+ shmem_fd = memfd_create("test", 0);
+ assert(shmem_fd >= 0);
+
+ ret = ftruncate(shmem_fd, page_size * 2);
+ assert(ret == 0);
+
+ buffer = mmap(NULL, page_size * 2, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE, shmem_fd, 0);
+ assert(buffer != MAP_FAILED);
+
+ /* Write private page, swap it out */
+ buffer[page_size] = 1;
+ madvise(buffer, page_size * 2, MADV_PAGEOUT);
+
+ /* This should drop private buffer[page_size] already */
+ ret = ftruncate(shmem_fd, page_size);
+ assert(ret == 0);
+ /* Recover the size */
+ ret = ftruncate(shmem_fd, page_size * 2);
+ assert(ret == 0);
+
+ /* Re-read the data, it should be all zero */
+ val = buffer[page_size];
+ if (val == 0)
+ printf("Good\n");
+ else
+ printf("BUG\n");
+ }
+===8<===
+
+We don't need to touch up the pmd path, because pmd never had a issue with
+swap entries. For example, shmem pmd migration will always be split into
+pte level, and same to swapping on anonymous.
+
+Add another helper should_zap_cows() so that we can also check whether we
+should zap private mappings when there's no page pointer specified.
+
+This patch drops that trick, so we handle swap ptes coherently. Meanwhile
+we should do the same check upon migration entry, hwpoison entry and
+genuine swap entries too.
+
+To be explicit, we should still remember to keep the private entries if
+even_cows==false, and always zap them when even_cows==true.
+
+The issue seems to exist starting from the initial commit of git.
+
+[peterx@redhat.com: comment tweaks]
+ Link: https://lkml.kernel.org/r/20220217060746.71256-2-peterx@redhat.com
+
+Link: https://lkml.kernel.org/r/20220217060746.71256-1-peterx@redhat.com
+Link: https://lkml.kernel.org/r/20220216094810.60572-1-peterx@redhat.com
+Link: https://lkml.kernel.org/r/20220216094810.60572-2-peterx@redhat.com
+Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
+Signed-off-by: Peter Xu <peterx@redhat.com>
+Reviewed-by: John Hubbard <jhubbard@nvidia.com>
+Cc: David Hildenbrand <david@redhat.com>
+Cc: Hugh Dickins <hughd@google.com>
+Cc: Alistair Popple <apopple@nvidia.com>
+Cc: Andrea Arcangeli <aarcange@redhat.com>
+Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
+Cc: Matthew Wilcox <willy@infradead.org>
+Cc: Vlastimil Babka <vbabka@suse.cz>
+Cc: Yang Shi <shy828301@gmail.com>
+Cc: <stable@vger.kernel.org>
+Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
+Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ mm/memory.c | 25 +++++++++++++++++++------
+ 1 file changed, 19 insertions(+), 6 deletions(-)
+
+--- a/mm/memory.c
++++ b/mm/memory.c
+@@ -1306,6 +1306,17 @@ int copy_page_range(struct mm_struct *ds
+ return ret;
+ }
+
++/* Whether we should zap all COWed (private) pages too */
++static inline bool should_zap_cows(struct zap_details *details)
++{
++ /* By default, zap all pages */
++ if (!details)
++ return true;
++
++ /* Or, we zap COWed pages only if the caller wants to */
++ return !details->check_mapping;
++}
++
+ static unsigned long zap_pte_range(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, pmd_t *pmd,
+ unsigned long addr, unsigned long end,
+@@ -1394,17 +1405,19 @@ again:
+ continue;
+ }
+
+- /* If details->check_mapping, we leave swap entries. */
+- if (unlikely(details))
+- continue;
+-
+ entry = pte_to_swp_entry(ptent);
+- if (!non_swap_entry(entry))
++ if (!non_swap_entry(entry)) {
++ /* Genuine swap entry, hence a private anon page */
++ if (!should_zap_cows(details))
++ continue;
+ rss[MM_SWAPENTS]--;
+- else if (is_migration_entry(entry)) {
++ } else if (is_migration_entry(entry)) {
+ struct page *page;
+
+ page = migration_entry_to_page(entry);
++ if (details && details->check_mapping &&
++ details->check_mapping != page_rmapping(page))
++ continue;
+ rss[mm_counter(page)]--;
+ }
+ if (unlikely(!free_swap_and_cache(entry)))
projects / thirdparty / kernel / stable-queue.git / commitdiff
