mm/madvise: fix data loss from MADV_FREE on a KSM page

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>

0 files changed, 0 insertions, 0 deletions