mm: zswap: always shrink in zswap_store() if zswap_pool_reached_full

mm: zswap: always shrink in zswap_store() if zswap_pool_reached_full

Patch series "zswap same-filled and limit checking cleanups", v3.

Miscellaneous cleanups for limit checking and same-filled handling in the
store path.  This series was broken out of the "zswap: store zero-filled
pages more efficiently" series [1].  It contains the cleanups and drops
the main functional changes.

[1]https://lore.kernel.org/lkml/20240325235018.2028408-1-yosryahmed@google.com/

This patch (of 4):

The cleanup code in zswap_store() is not pretty, particularly the 'shrink'
label at the bottom that ends up jumping between cleanup labels.

Instead of having a dedicated label to shrink the pool, just use
zswap_pool_reached_full directly to figure out if the pool needs
shrinking.  zswap_pool_reached_full should be true if and only if the pool
needs shrinking.

The only caveat is that the value of zswap_pool_reached_full may be
changed by concurrent zswap_store() calls between checking the limit and
testing zswap_pool_reached_full in the cleanup code.  This is fine
because:

- If zswap_pool_reached_full was true during limit checking then became
  false during the cleanup code, then someone else already took care of
  shrinking the pool and there is no need to queue the worker. That
  would be a good change.
- If zswap_pool_reached_full was false during limit checking then became
  true during the cleanup code, then someone else hit the limit
  meanwhile. In this case, both threads will try to queue the worker,
  but it never gets queued more than once anyway. Also, calling
  queue_work() multiple times when the limit is hit could already happen
  today, so this isn't a significant change in any way.

Link: https://lkml.kernel.org/r/20240413022407.785696-1-yosryahmed@google.com
Link: https://lkml.kernel.org/r/20240413022407.785696-2-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>