Re: [PATCH v3 41/47] mm/ksm: check large swap-cache folios before mapping them

[Lance Yang]

On Sat, Oct 31, 2026 at 10:40:10AM +0100, David Hildenbrand (Arm) wrote:
>From: Chris Mason <mason@xxxxxxxxxx>
>
>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@xxxxxxxxxxxxxxx
>Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
>Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
>Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>---

LGTM1

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>