mm, swap: check page poison flag after locking it

mm, swap: check page poison flag after locking it

Instead of checking the poison flag only in the fast swap cache lookup
path, always check the poison flags after locking a swap cache folio.

There are two reasons to do so.

The folio is unstable and could be removed from the swap cache anytime, so
it's totally possible that the folio is no longer the backing folio of a
swap entry, and could be an irrelevant poisoned folio.  We might
mistakenly kill a faulting process.

And it's totally possible or even common for the slow swap in path
(swapin_readahead) to bring in a cached folio.  The cache folio could be
poisoned, too.  Only checking the poison flag in the fast path will miss
such folios.

The race window is tiny, so it's very unlikely to happen, though.  While
at it, also add a unlikely prefix.

Link: https://lkml.kernel.org/r/20250910160833.3464-5-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>