mm, swap: clean up and improve swap entries freeing

mm, swap: clean up and improve swap entries freeing

There are a few problems with the current freeing of swap entries.

When freeing a set of swap entries directly (swap_put_entries_direct,
typically from zapping the page table), it scans the whole swap region
multiple times.  First, it scans the whole region to check if it can be
batch freed and if there is any cached folio.  Then do a batch free only
if the whole region's swap count equals 1.  And if any entry is cached,
even if only one, it will have to walk the whole region again to clean up
the cache.

And if any entry is not in a consistent status with other entries, it will
fall back to order 0 freeing.  For example, if only one of them is cached,
the batch free will fall back.

And the current batch freeing workflow relies on the swap map's
SWAP_HAS_CACHE bit for both continuous checking and batch freeing, which
isn't compatible with the swap table design.

Tidy this up, introduce a new cluster scoped helper for all swap entry
freeing job.  It will batch frees all continuous entries, and just start a
new batch if any inconsistent entry is found.  This may improve the batch
size when the clusters are fragmented.  This should also be more robust
with more sanity checks, and make it clear that a slot pinned by swap
cache will be cleared upon cache reclaim.

And the cache reclaim scan is also now limited to each cluster.  If a
cluster has any clean swap cache left after putting the swap count,
reclaim the cluster only instead of the whole region.

And since a folio's entries are always in the same cluster, putting swap
entries from a folio can also use the new helper directly.

This should be both an optimization and a cleanup, and the new helper is
adapted to the swap table.

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-17-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@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>