[PATCH v3] mm/migrate: fix shmem xarray update during migration

Zi Yan
2025-03-05T20:04:17Z

A shmem folio can be either in page cache or in swap cache, but not at the
same time. Namely, once it is in swap cache, folio->mapping should be NULL,
and the folio is no longer in a shmem mapping.

In __folio_migrate_mapping(), to determine the number of xarray entries
to update, folio_test_swapbacked() is used, but that conflates shmem in
page cache case and shmem in swap cache case. It leads to xarray
multi-index entry corruption, since it turns a sibling entry to a
normal entry during xas_store() (see [1] for a userspace reproduction).
Fix it by only using folio_test_swapcache() to determine whether xarray
is storing swap cache entries or not to choose the right number of xarray
entries to update.

[1] https://lore.kernel.org/linux-mm/Z8idPCkaJW1IChjT@casper.infradead.org/

Note:
In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is used
to get swap_cache address space, but that ignores the shmem folio in swap
cache case. It could lead to NULL pointer dereferencing when a
in-swap-cache shmem folio is split at __xa_store(), since
!folio_test_anon() is true and folio->mapping is NULL. But fortunately,
its caller split_huge_page_to_list_to_order() bails out early with EBUSY
when folio->mapping is NULL. So no need to take care of it here.

Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
Reported-by: Liu Shixin <liushixin2@huawei.com>
Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: stable@vger.kernel.org
---
mm/migrate.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index fb4afd31baf0..c0adea67cd62 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -518,15 +518,13 @@ static int __folio_migrate_mapping(struct address_space *mapping,
if (folio_test_anon(folio) && folio_test_large(folio))
mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
folio_ref_add(newfolio, nr); /* add cache reference */
- if (folio_test_swapbacked(folio)) {
+ if (folio_test_swapbacked(folio))
__folio_set_swapbacked(newfolio);
- if (folio_test_swapcache(folio)) {
- folio_set_swapcache(newfolio);
- newfolio->private = folio_get_private(folio);
- }
+ if (folio_test_swapcache(folio)) {
+ folio_set_swapcache(newfolio);
+ newfolio->private = folio_get_private(folio);
entries = nr;
} else {
- VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
entries = 1;
}

--
2.47.2