[PATCH v3] zswap: do not shrink if cgroup may not zswap

[Nhat Pham <nphamcs@gmail.com>]

Before storing a page, zswap first checks if the number of stored pages
exceeds the limit specified by memory.zswap.max, for each cgroup in the
hierarchy. If this limit is reached or exceeded, then zswap shrinking is
triggered and short-circuits the store attempt.

However, since the zswap's LRU is not memcg-aware, this can create the
following pathological behavior: the cgroup whose zswap limit is 0 will
evict pages from other cgroups continually, without lowering its own
zswap usage. This means the shrinking will continue until the need for
swap ceases or the pool becomes empty.

As a result of this, we observe a disproportionate amount of zswap
writeback and a perpetually small zswap pool in our experiments, even
though the pool limit is never hit.

More generally, a cgroup might unnecessarily evict pages from other
cgroups before we drive the memcg back below its limit.

This patch fixes the issue by rejecting zswap store attempt without
shrinking the pool when obj_cgroup_may_zswap() returns false.

Fixes: f4840ccfca25 ("zswap: memcg accounting")
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zswap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 59da2a415fbb..cff93643a6ab 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1174,9 +1174,14 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 		goto reject;
 	}
 
+	/*
+	 * XXX: zswap reclaim does not work with cgroups yet. Without a
+	 * cgroup-aware entry LRU, we will push out entries system-wide based on
+	 * local cgroup limits.
+	 */
 	objcg = get_obj_cgroup_from_page(page);
 	if (objcg && !obj_cgroup_may_zswap(objcg))
-		goto shrink;
+		goto reject;
 
 	/* reclaim space if needed */
 	if (zswap_is_full()) {
-- 
2.34.1