[PATCH 40/46] mm/ksm: check large swap-cache folios before mapping them

[sashiko-bot at kernel.org]

> From: Chris Mason <mason at kernel.org>
> 
> KSM folios are always small, but their shared swap slots can be read
> back as part of a large folio in the swap cache. ksm_might_need_to_copy()
> skips large folios, so another process can map a page with an incompatible
> anon_vma or page index. Reverse mapping then cannot find the new
> mapping, preventing migration or reclaim.
> 
> Apply the same compatibility checks to large folios as to small ones.
> Pass the page corresponding to the swap entry and copy only that page
> when necessary. In do_swap_page()'s copy path, decrement only that slot's
> swap count, because the source swap-cache folio can now be large.
> 
> Fix generated by an LLM and sent by Chris Mason in private.
> 
> Fixes: f1879e8a0c60 ("mm, swap: never bypass the swap cache even for SWP_SYNCHRONOUS_IO")
> Cc: stable at vger.kernel.org
> Signed-off-by: Chris Mason <mason 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=40