mm/ksm: check large swap-cache folios before mapping them

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@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-41-d6437171ecc3@kernel.org
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

0 files changed, 0 insertions, 0 deletions