[PATCH 33/46] mm/madvise: fix data loss from MADV_FREE on a KSM page

[David Hildenbrand (Arm)]

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 at vger.kernel.org
Reported-by: Josef Bacik <josef at toxicpanda.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/madvise.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index 75b409e97dbf2..a16e3cb256b7b 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -756,6 +756,15 @@ 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 we have a large folio at this point, we know it is
 			 * fully mapped so if its mapcount is the same as its

-- 
2.43.0