mm, swap: drop the SWAP_HAS_CACHE flag

mm, swap: drop the SWAP_HAS_CACHE flag

Now, the swap cache is managed by the swap table.  All swap cache users
are checking the swap table directly to check the swap cache state.
SWAP_HAS_CACHE is now just a temporary pin before the first increase from
0 to 1 of a slot's swap count (swap_dup_entries) after swap allocation
(folio_alloc_swap), or before the final free of slots pinned by folio in
swap cache (put_swap_folio).

Drop these two usages.  For the first dup, SWAP_HAS_CACHE pinning was hard
to kill because it used to have multiple meanings, more than just "a slot
is cached".  We have just simplified that and defined that the first dup
is always done with folio locked in swap cache (folio_dup_swap), so stop
checking the SWAP_HAS_CACHE bit and just check the swap cache (swap table)
directly, and add a WARN if a swap entry's count is being increased for
the first time while the folio is not in swap cache.

As for freeing, just let the swap cache free all swap entries of a folio
that have a swap count of zero directly upon folio removal.  We have also
just cleaned up batch freeing to check the swap cache usage using the swap
table: a slot with swap cache in the swap table will not be freed until
its cache is gone, and no SWAP_HAS_CACHE bit is involved anymore.  And
besides, the removal of a folio and freeing of the slots are being done in
the same critical section now, which should improve the performance.

After these two changes, SWAP_HAS_CACHE no longer has any users.  Swap
cache synchronization is also done by the swap table directly, so using
SWAP_HAS_CACHE to pin a slot before adding the cache is also no longer
needed.  Remove all related logic and helpers.  swap_map is now only used
for tracking the count, so all swap_map users can just read it directly,
ignoring the swap_count helper, which was previously used to filter out
the SWAP_HAS_CACHE bit.

The idea of dropping SWAP_HAS_CACHE and using the swap table directly was
initially from Chris's idea of merging all the metadata usage of all swaps
into one place.

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-18-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>