From: Yosry Ahmed <yosryahmed@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Nhat Pham <nphamcs@gmail.com>,
hannes@cmpxchg.org, cerasuolodomenico@gmail.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
sjenning@redhat.com, ddstreet@ieee.org,
vitaly.wool@konsulko.com, kernel-team@meta.com
Subject: Re: [PATCH] zswap: do not shrink when memory.zswap.max is 0
Date: Wed, 7 Jun 2023 12:17:09 -0700 [thread overview]
Message-ID: <CAJD7tkbsy7X08zp8FF-XYn2krd6hDgWVz7t2ZoFr+RAm_Zcn4w@mail.gmail.com> (raw)
In-Reply-To: <20230607120939.c1256fac4bb4e081811c02da@linux-foundation.org>
On Wed, Jun 7, 2023 at 12:09 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> It's unclear (to me) whether we should proceed with this. Thoughts, please?
This version looks good to me. I added my Reviewed-by on some version,
but I guess it got lost with all the versions and fixlets :)
>
> Here's what I presently have in mm-hotfixes-unstable:
>
>
> 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 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>
Reviewed-by: Yosry Ahmed <yosryahmed@google.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>
> ---
>
> mm/zswap.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> --- a/mm/zswap.c~zswap-do-not-shrink-if-cgroup-may-not-zswap
> +++ a/mm/zswap.c
> @@ -1174,9 +1174,16 @@ 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()) {
> _
>
next prev parent reply other threads:[~2023-06-07 19:17 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-30 16:21 [PATCH] zswap: do not shrink when memory.zswap.max is 0 Nhat Pham
2023-05-30 16:52 ` Yosry Ahmed
2023-05-30 18:00 ` Johannes Weiner
2023-05-30 18:41 ` Yosry Ahmed
2023-05-30 19:13 ` Johannes Weiner
2023-05-30 20:19 ` Yosry Ahmed
2023-05-30 20:59 ` Johannes Weiner
2023-05-30 21:04 ` Yosry Ahmed
2023-05-30 21:46 ` Nhat Pham
2023-05-30 22:24 ` [PATCH] zswap: do not shrink if cgroup may not zswap Nhat Pham
2023-05-30 22:30 ` Andrew Morton
2023-05-30 23:37 ` Nhat Pham
2023-05-30 22:36 ` Yosry Ahmed
2023-05-30 23:24 ` [PATCH v3] " Nhat Pham
2023-05-30 18:27 ` [PATCH] zswap: do not shrink when memory.zswap.max is 0 Nhat Pham
2023-05-30 18:42 ` Yosry Ahmed
2023-06-07 19:09 ` Andrew Morton
2023-06-07 19:17 ` Yosry Ahmed [this message]
2023-06-07 19:31 ` Nhat Pham
2023-06-07 20:42 ` Johannes Weiner
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