mm: zswap: remove nr_zswap_stored atomic

nr_stored was introduced by commit b5ba474f ("zswap: shrink zswap pool
based on memory pressure") as a per zswap_pool counter of the number of
stored pages that are not same-filled pages.  It is used in
zswap_shrinker_count() to scale the number of freeable compressed pages by
the compression ratio.  That is, to reduce the amount of writeback from
zswap with higher compression ratios as the ROI from IO diminishes.

Later on, commit bf9b7df2 ("mm/zswap: global lru and shrinker shared
by all zswap_pools") made the shrinker global (not per zswap_pool), and
replaced nr_stored with nr_zswap_stored (initially introduced as
zswap.nr_stored), which is now a global counter.

The counter is now awfully close to zswap_stored_pages.  The only
difference is that the latter also includes same-filled pages.  Also, when
memcgs are enabled, we use memcg_page_state(memcg, MEMCG_ZSWAPPED), which
includes same-filled pages anyway (i.e.  equivalent to
zswap_stored_pages).

Use zswap_stored_pages instead in zswap_shrinker_count() to keep things
consistent whether memcgs are enabled or not, and add a comment about the
number of freeable pages possibly being scaled down more than it should if
we have lots of same-filled pages (i.e.  inflated compression ratio).

Remove nr_zswap_stored and one atomic operation in the store and free
paths.

Link: https://lkml.kernel.org/r/20240322001001.1562517-1-yosryahmed@google.com


Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>