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.comSigned-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>

mm/zswap.c

@@ -181,8 +181,6 @@ struct zswap_pool {
 
 /* Global LRU lists shared by all zswap pools. */
 static struct list_lru zswap_list_lru;
-/* counter of pages stored in all zswap pools. */
-static atomic_t zswap_nr_stored = ATOMIC_INIT(0);
 
 /* The lock protects zswap_next_shrink updates. */
 static DEFINE_SPINLOCK(zswap_shrink_lock);
@@ -885,7 +883,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
 	else {
 		zswap_lru_del(&zswap_list_lru, entry);
 		zpool_free(zswap_find_zpool(entry), entry->handle);
-		atomic_dec(&zswap_nr_stored);
 		zswap_pool_put(entry->pool);
 	}
 	if (entry->objcg) {
@@ -1325,7 +1322,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 		nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
 	} else {
 		nr_backing = zswap_total_pages();
-		nr_stored = atomic_read(&zswap_nr_stored);
+		nr_stored = atomic_read(&zswap_stored_pages);
 	}
 
 	if (!nr_stored)
@@ -1345,6 +1342,11 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 	 * This ensures that the better zswap compresses memory, the fewer
 	 * pages we will evict to swap (as it will otherwise incur IO for
 	 * relatively small memory saving).
+	 *
+	 * The memory saving factor calculated here takes same-filled pages into
+	 * account, but those are not freeable since they almost occupy no
+	 * space. Hence, we may scale nr_freeable down a little bit more than we
+	 * should if we have a lot of same-filled pages.
 	 */
 	return mult_frac(nr_freeable, nr_backing, nr_stored);
 }
@@ -1590,7 +1592,6 @@ bool zswap_store(struct folio *folio)
 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
 		zswap_lru_add(&zswap_list_lru, entry);
-		atomic_inc(&zswap_nr_stored);
 	}
 	spin_unlock(&tree->lock);