linux-kernel - Re: [PATCH v6] zswap: memcontrol: implement zswap writeback disabling (fix)

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Date: Sun, 24 Dec 2023 09:17:54 -0800
From: Chris Li <chriscli@...gle.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: akpm@...ux-foundation.org, tj@...nel.org, lizefan.x@...edance.com, 
	hannes@...xchg.org, cerasuolodomenico@...il.com, yosryahmed@...gle.com, 
	sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com, 
	mhocko@...nel.org, roman.gushchin@...ux.dev, shakeelb@...gle.com, 
	muchun.song@...ux.dev, hughd@...gle.com, corbet@....net, 
	konrad.wilk@...cle.com, senozhatsky@...omium.org, rppt@...nel.org, 
	linux-mm@...ck.org, kernel-team@...a.com, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, david@...t.cz
Subject: Re: [PATCH v6] zswap: memcontrol: implement zswap writeback disabling (fix)

Hi Nhat,

Acked-by: Chris Li <chrisl@...nel.org>

I think a follow up step would be having some patches to address it
rather than document it that oh yes, we have a problem in that
situation.

Chris

On Wed, Dec 20, 2023 at 4:57 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> Add a caveat about recurring zswap store failures leading to reclaim
> inefficiency.
>
> Suggested-by: Yosry Ahmed <yosryahmed@...gle.com>
> Signed-off-by: Nhat Pham <nphamcs@...il.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 5 ++++-
>  Documentation/admin-guide/mm/zswap.rst  | 4 ++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 2b4ac43efdc8..5ec7dd753cd1 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1686,7 +1686,10 @@ PAGE_SIZE multiple when read back.
>
>         When this is set to 0, all swapping attempts to swapping devices
>         are disabled. This included both zswap writebacks, and swapping due
> -       to zswap store failure.
> +       to zswap store failures. If the zswap store failures are recurring
> +       (for e.g if the pages are incompressible), users can observe
> +       reclaim inefficiency after disabling writeback (because the same
> +       pages might be rejected again and again).
>
>         Note that this is subtly different from setting memory.swap.max to
>         0, as it still allows for pages to be written to the zswap pool.
> diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
> index cfa653130346..b42132969e31 100644
> --- a/Documentation/admin-guide/mm/zswap.rst
> +++ b/Documentation/admin-guide/mm/zswap.rst
> @@ -159,6 +159,10 @@ zswap itself) on a cgroup-basis as follows:
>
>         echo 0 > /sys/fs/cgroup/<cgroup-name>/memory.zswap.writeback
>
> +Note that if the store failures are recurring (for e.g if the pages are
> +incompressible), users can observe reclaim inefficiency after disabling
> +writeback (because the same pages might be rejected again and again).
> +
>  When there is a sizable amount of cold memory residing in the zswap pool, it
>  can be advantageous to proactively write these cold pages to swap and reclaim
>  the memory for other use cases. By default, the zswap shrinker is disabled.
> --
> 2.34.1
>