The patch titled
Subject: mm/migrate: fix shmem xarray update during migration
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-migrate-fix-shmem-xarray-update-during-migration.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-migrate-fix-shmem-xarray-update-during-migration.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Zi Yan <ziy@xxxxxxxxxx>
Subject: mm/migrate: fix shmem xarray update during migration
Date: Wed, 5 Mar 2025 15:04:03 -0500
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@xxxxxxxxxxxxxxxxxxxx/
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.
Link: https://lkml.kernel.org/r/20250305200403.2822855-1-ziy@xxxxxxxxxx
Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
Reported-by: Liu Shixin <liushixin2@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@xxxxxxxxxx/
Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Cc: Lance Yang <ioworker0@xxxxxxxxx>
Cc: Matthew Wilcow (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/migrate.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)
--- a/mm/migrate.c~mm-migrate-fix-shmem-xarray-update-during-migration
+++ a/mm/migrate.c
@@ -518,15 +518,13 @@ static int __folio_migrate_mapping(struc
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;
}
_
Patches currently in -mm which might be from ziy@xxxxxxxxxx are
mm-migrate-fix-shmem-xarray-update-during-migration.patch
selftests-mm-make-file-backed-thp-split-work-by-writing-pmd-size-data.patch
mm-huge_memory-allow-split-shmem-large-folio-to-any-lower-order.patch
selftests-mm-test-splitting-file-backed-thp-to-any-lower-order.patch
