mm, swap: never bypass the swap cache even for SWP_SYNCHRONOUS_IO

mm, swap: never bypass the swap cache even for SWP_SYNCHRONOUS_IO

Now the overhead of the swap cache is trivial.  Bypassing the swap cache
is no longer a valid optimization.  So unify the swapin path using the
swap cache.  This changes the swap in behavior in two observable ways.

Readahead is now always disabled for SWP_SYNCHRONOUS_IO devices, which is
a huge win for some workloads: We used to rely on `SWP_SYNCHRONOUS_IO &&
__swap_count(entry) == 1` as the indicator to bypass both the swap cache
and readahead, the swap count check made bypassing ineffective in many
cases, and it's not a good indicator.  The limitation existed because the
current swap design made it hard to decouple readahead bypassing and swap
cache bypassing.  We do want to always bypass readahead for
SWP_SYNCHRONOUS_IO devices, but bypassing swap cache at the same time will
cause repeated IO and memory overhead.  Now that swap cache bypassing is
gone, this swap count check can be dropped.

The second thing here is that this enabled large swapin for all swap
entries on SWP_SYNCHRONOUS_IO devices.  Previously, the large swap in is
also coupled with swap cache bypassing, and so the swap count checking
also makes large swapin less effective.  Now this is also improved.  We
will always have large swapin supported for all SWP_SYNCHRONOUS_IO cases.

And to catch potential issues with large swapin, especially with page
exclusiveness and swap cache, more debug sanity checks and comments are
added.  But overall, the code is simpler.  And new helper and routines
will be used by other components in later commits too.  And now it's
possible to rely on the swap cache layer for resolving synchronization
issues, which will also be done by a later commit.

Worth mentioning that for a large folio workload, this may cause more
serious thrashing.  This isn't a problem with this commit, but a generic
large folio issue.  For a 4K workload, this commit increases the
performance.

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-3-8862a265a033@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>