mm, swap: simplify checking if a folio is swapped

mm, swap: simplify checking if a folio is swapped

Clean up and simplify how we check if a folio is swapped.  The helper
already requires the folio to be in swap cache and locked.  That's enough
to pin the swap cluster from being freed, so there is no need to lock
anything else to avoid UAF.

And besides, we have cleaned up and defined the swap operation to be
mostly folio based, and now the only place a folio will have any of its
swap slots' count increased from 0 to 1 is folio_dup_swap, which also
requires the folio lock.  So as we are holding the folio lock here, a
folio can't change its swap status from not swapped (all swap slots have a
count of 0) to swapped (any slot has a swap count larger than 0).

So there won't be any false negatives of this helper if we simply depend
on the folio lock to stabilize the cluster.

We are only using this helper to determine if we can and should release
the swap cache.  So false positives are completely harmless, and also
already exist before.  Depending on the timing, previously, it's also
possible that a racing thread releases the swap count right after
releasing the ci lock and before this helper returns.  In any case, the
worst that could happen is we leave a clean swap cache.  It will still be
reclaimed when under pressure just fine.

So, in conclusion, we can simplify and make the check much simpler and
lockless.  Also, rename it to folio_maybe_swapped to reflect the design.

Link: https://lkml.kernel.org/r/20260218-swap-table-p3-v3-11-f4e34be021a7@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>