mm/khugepaged: flush deferred unmaps before dropping a failed folio

collapse_file() can fail its reference-count or dirty-folio check after
unmapping with TTU_BATCH_FLUSH.  Both paths put back the isolated folio,
then unlock it and drop the lookup reference before reaching the common
try_to_unmap_flush().

The page-cache reference does not keep the folio stable once the lock is
released.  Another collapse can replace and free it.  With its PTEs
already gone, that collapse cannot flush the first task's per-task TLB
batch, and retract_page_tables() skips short or unaligned VMAs.  A CPU can
therefore retain a user translation to the freed folio.  This has been
reproduced with unprivileged MADV_COLLAPSE on a memfd.

Flush at out_unlock while the lookup reference and folio lock are still
held.  The common flush continues to cover the accumulated pagelist on
both success and rollback, preserving batching on successful collapses.

Link: https://lore.kernel.org/20261007041001.43181-1-kylebot@openai.com
Fixes: 6d9df8a5889c ("mm/thp: collapse_file() do try_to_unmap(TTU_BATCH_FLUSH)")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Assisted-by: LLM
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions