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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 7 Dec 2023 11:26:58 -0800
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Subject: Re: [PATCH v6] zswap: memcontrol: implement zswap writeback disabling
To:     Nhat Pham <nphamcs@gmail.com>
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On Thu, Dec 7, 2023 at 11:24=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> wrote=
:
>
> During our experiment with zswap, we sometimes observe swap IOs due to
> occasional zswap store failures and writebacks-to-swap. These swapping
> IOs prevent many users who cannot tolerate swapping from adopting zswap
> to save memory and improve performance where possible.
>
> This patch adds the option to disable this behavior entirely: do not
> writeback to backing swapping device when a zswap store attempt fail,
> and do not write pages in the zswap pool back to the backing swap
> device (both when the pool is full, and when the new zswap shrinker is
> called).
>
> This new behavior can be opted-in/out on a per-cgroup basis via a new
> cgroup file. By default, writebacks to swap device is enabled, which is
> the previous behavior. Initially, writeback is enabled for the root
> cgroup, and a newly created cgroup will inherit the current setting of
> its parent.
>
> Note that this is subtly different from setting memory.swap.max to 0, as
> it still allows for pages to be stored in the zswap pool (which itself
> consumes swap space in its current form).
>
> This patch should be applied on top of the zswap shrinker series:
>
> https://lore.kernel.org/linux-mm/20231130194023.4102148-1-nphamcs@gmail.c=
om/
>
> as it also disables the zswap shrinker, a major source of zswap
> writebacks.
>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

FTR I still prefer a more generic and future-proof interface (e.g.
memory.swap.tiers), but I am not opposed to this. It will just be
annoying to have different interfaces with overlapping functionalities
in the future if/when the need for a generic interface arises.