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

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, May 30, 2023 at 3:24 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> 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.

This pathological behavior will only happen if the zswap limit is 0.
Otherwise, we will see a different pathological behavior where we
unnecessarily evict X pages from other cgroups before we drive the
memcg back below its limit.

Perhaps we should clarify this?

>
> 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.

I am guessing this is also related to the case where the limit is 0.
It would be useful to clarify this.

>
> 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")
> 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
>

With commit log nits above:

Reviewed-by: Yosry Ahmed <yosryahmed@google.com>