Re: [PATCH] zswap: do not shrink when memory.zswap.max is 0

[Nhat Pham]

On Wed, Jun 7, 2023 at 12:09 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> It's unclear (to me) whether we should proceed with this.  Thoughts, please?
>
> Here's what I presently have in mm-hotfixes-unstable:
>
>
> From: Nhat Pham <nphamcs@xxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxxx: fix return of unintialized value]
> [akpm@xxxxxxxxxxxxxxxxxxxx: s/ENOSPC/ENOMEM/]
> Link: https://lkml.kernel.org/r/20230530222440.2777700-1-nphamcs@xxxxxxxxx
> Link: https://lkml.kernel.org/r/20230530232435.3097106-1-nphamcs@xxxxxxxxx
> Fixes: f4840ccfca25 ("zswap: memcg accounting")
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: Dan Streetman <ddstreet@xxxxxxxx>
> Cc: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Seth Jennings <sjenning@xxxxxxxxxx>
> Cc: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
>  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()) {
> _
>

Apologies for lack of clarity - yep this is the final version
I had in mind too.