+ zswap-do-not-shrink-if-cgroup-may-not-zswap.patch added to mm-unstable branch

+ zswap-do-not-shrink-if-cgroup-may-not-zswap.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: zswap: do not shrink if cgroup may not zswap
has been added to the -mm mm-unstable branch.  Its filename is
     zswap-do-not-shrink-if-cgroup-may-not-zswap.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zswap-do-not-shrink-if-cgroup-may-not-zswap.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nhat Pham <nphamcs@gmail.com>
Subject: zswap: do not shrink if cgroup may not zswap
Date: Tue, 30 May 2023 15:24:40 -0700

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
reached 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.

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

Link: https://lkml.kernel.org/r/20230530222440.2777700-1-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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/zswap.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/mm/zswap.c~zswap-do-not-shrink-if-cgroup-may-not-zswap
+++ a/mm/zswap.c
@@ -1174,9 +1174,14 @@ 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;
+		goto reject;
 
 	/* reclaim space if needed */
 	if (zswap_is_full()) {
_

Patches currently in -mm which might be from nphamcs@gmail.com are

workingset-refactor-lru-refault-to-expose-refault-recency-check.patch
cachestat-implement-cachestat-syscall.patch
cachestat-implement-cachestat-syscall-fix.patch
cachestat-wire-up-cachestat-for-other-architectures.patch
cachestat-wire-up-cachestat-for-other-architectures-fix.patch
selftests-add-selftests-for-cachestat.patch
zswap-do-not-shrink-if-cgroup-may-not-zswap.patch