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

[sashiko-bot at kernel.org]

> 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>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org?part=33