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From: Chris Li <chriscli@google.com>
Date: Sun, 24 Dec 2023 09:17:54 -0800
Message-ID: <CAF8kJuMQhbkR2p0UZ61r+tAGg-idiSMoW8Gsf=GcB3Jpa=W9qw@mail.gmail.com>
Subject: Re: [PATCH v6] zswap: memcontrol: implement zswap writeback disabling (fix)
To: Nhat Pham <nphamcs@gmail.com>
Cc: akpm@linux-foundation.org, tj@kernel.org, lizefan.x@bytedance.com, 
	hannes@cmpxchg.org, cerasuolodomenico@gmail.com, yosryahmed@google.com, 
	sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com, 
	mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com, 
	muchun.song@linux.dev, hughd@google.com, corbet@lwn.net, 
	konrad.wilk@oracle.com, senozhatsky@chromium.org, rppt@kernel.org, 
	linux-mm@kvack.org, kernel-team@meta.com, linux-kernel@vger.kernel.org, 
	linux-doc@vger.kernel.org, david@ixit.cz
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Hi Nhat,

Acked-by: Chris Li <chrisl@kernel.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=E2=80=AFPM Nhat Pham <nphamcs@gmail.com> wrote=
:
>
> Add a caveat about recurring zswap store failures leading to reclaim
> inefficiency.
>
> Suggested-by: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Nhat Pham <nphamcs@gmail.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/admi=
n-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 d=
ue
> -       to zswap store failure.
> +       to zswap store failures. If the zswap store failures are recurrin=
g
> +       (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 t=
o
>         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 re=
claim
>  the memory for other use cases. By default, the zswap shrinker is disabl=
ed.
> --
> 2.34.1
>