From: Nhat Pham nphamcs@gmail.com
commit 0bdf0efa180a9cb1361cbded4e2260a49306ac89 upstream.
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.
[akpm@linux-foundation.org: fix return of unintialized value] [akpm@linux-foundation.org: s/ENOSPC/ENOMEM/] Link: https://lkml.kernel.org/r/20230530222440.2777700-1-nphamcs@gmail.com Link: https://lkml.kernel.org/r/20230530232435.3097106-1-nphamcs@gmail.com Fixes: f4840ccfca25 ("zswap: memcg accounting") Signed-off-by: Nhat Pham nphamcs@gmail.com Cc: Dan Streetman ddstreet@ieee.org Cc: Domenico Cerasuolo cerasuolodomenico@gmail.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Seth Jennings sjenning@redhat.com Cc: Vitaly Wool vitaly.wool@konsulko.com Cc: Yosry Ahmed yosryahmed@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/zswap.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
--- a/mm/zswap.c +++ b/mm/zswap.c @@ -1138,9 +1138,16 @@ static int zswap_frontswap_store(unsigne 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; + if (objcg && !obj_cgroup_may_zswap(objcg)) { + ret = -ENOMEM; + goto reject; + }
/* reclaim space if needed */ if (zswap_is_full()) {