[PATCH 6.6.y] mm/madvise: fix data loss from MADV_FREE on a KSM page

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit 22d043c1da48d6c7274f9dd900714b6d58b7b9c6 ]

MADV_FREE can clear the dirty flag of a singly mapped KSM folio. The
folio remains in the stable tree and can gain new mappings without being
dirtied again. Once marked lazyfree, reclaim can discard data that those
mappings still need.

KSM always marks a folio dirty when converting it to a KSM folio in
try_to_merge_one_page(). Marking a folio lazyfree clears PG_swapbacked
but leaves PG_dirty unchanged. Preserving PG_dirty makes reclaim restore
PG_swapbacked instead of discarding the folio. This also protects folios
that were already lazyfree when KSM converted them.

Skip KSM folios before clearing their dirty flag, checking under the
folio lock to serialize with KSM conversion.

Fixes: 854e9ed09ded ("mm: support madvise(MADV_FREE)")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Xu Xin <xu.xin@linux.dev>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-34-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: Adjusted surrounding comment context for the existing single-page folio mapcount check. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/madvise.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index 209d0cc950139..9640a13699f5d 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -710,6 +710,15 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 		if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
 			if (!folio_trylock(folio))
 				continue;
+			/*
+			 * KSM folios can gain new mappings at any time, so we must
+			 * not clear their dirty flag.
+			 */
+			if (folio_test_ksm(folio)) {
+				folio_unlock(folio);
+				continue;
+			}
+
 			/*
 			 * If folio is shared with others, we mustn't clear
 			 * the folio's dirty flag.
-- 
2.53.0