[PATCH v1 5/5] mm: zswap: do not check the global limit for same-filled pages

[Yosry Ahmed]

When storing same-filled pages, there is no point of checking the global
zswap limit as storing them does not contribute toward the limit Move
the limit checking after same-filled pages are handled.

This avoids having same-filled pages skip zswap and go to disk swap if
the limit is hit. It also avoids queueing the shrink worker, which may
end up being unnecessary if the zswap usage goes down on its own before
another store is attempted.

Ignoring the memcg limits as well for same-filled pages is more
controversial. Those limits are more a matter of per-workload policy.
Some workloads disable zswap completely by setting memory.zswap.max = 0,
and those workloads could start observing some zswap activity even after
disabling zswap. Although harmless, this could cause confusion to
userspace. Remain conservative and keep respecting those limits.

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 mm/zswap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index a85c9235d19d3..8763a1e938441 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1404,6 +1404,7 @@ bool zswap_store(struct folio *folio)
 	struct zswap_entry *entry, *old;
 	struct obj_cgroup *objcg = NULL;
 	struct mem_cgroup *memcg = NULL;
+	bool same_filled = false;
 	unsigned long value;
 
 	VM_WARN_ON_ONCE(!folio_test_locked(folio));
@@ -1427,7 +1428,8 @@ bool zswap_store(struct folio *folio)
 		mem_cgroup_put(memcg);
 	}
 
-	if (zswap_check_full())
+	same_filled = zswap_is_folio_same_filled(folio, &value);
+	if (!same_filled && zswap_check_full())
 		goto reject;
 
 	/* allocate entry */
@@ -1437,7 +1439,7 @@ bool zswap_store(struct folio *folio)
 		goto reject;
 	}
 
-	if (zswap_is_folio_same_filled(folio, &value)) {
+	if (same_filled) {
 		entry->length = 0;
 		entry->value = value;
 		atomic_inc(&zswap_same_filled_pages);
-- 
2.44.0.478.gd926399ef9-goog