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From:   Chris Li <chrisl@kernel.org>
Date:   Sun, 19 Nov 2023 01:39:26 -0800
Message-ID: <CAF8kJuOD6zq2VPcVdoZGvkzYX8iXn1akuYhNDJx-LUdS+Sx3GA@mail.gmail.com>
Subject: Re: [PATCH v5] zswap: memcontrol: implement zswap writeback disabling
To:     Nhat Pham <nphamcs@gmail.com>
Cc:     Yosry Ahmed <yosryahmed@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, tj@kernel.org,
        lizefan.x@bytedance.com, Johannes Weiner <hannes@cmpxchg.org>,
        Domenico Cerasuolo <cerasuolodomenico@gmail.com>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>, mhocko@kernel.org,
        roman.gushchin@linux.dev, Shakeel Butt <shakeelb@google.com>,
        muchun.song@linux.dev, Hugh Dickins <hughd@google.com>,
        corbet@lwn.net, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        senozhatsky@chromium.org, rppt@kernel.org,
        linux-mm <linux-mm@kvack.org>, kernel-team@meta.com,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        david@ixit.cz
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On Sat, Nov 18, 2023 at 11:23=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> wrot=
e:
>
> Hmm how about this - in the future, we support the following
> options:
>
> 1. zswap.writeback =3D=3D 1: no limitation to zswap writeback.
> All backing swap devices (sorted by priorities?) are fair game.
>
> 2. zswap.writeback =3D=3D 0: disable all forms of zswap writeback.
>
> 3. zswap.writeback =3D=3D <tiers description>:  attempt to write to each
> tier, one at a time.

We can merge the zswap.writeback as it is for now to unblock you.

For the future. I think we should remove zswap.writeback completely.

Instead we have:

swap.tiers =3D=3D <swap_tier_list_name>
swap.tiers =3D=3D "all" all available swap tiers. "zswap + swap file".
This is the default.
swap.tiers =3D=3D "zswap" zswap only, no other swap file. Internally set
zswap.writeback =3D 0
swap.tiers =3D=3D "foo" foo is a list of swap devices it can use. You can
define your town custom swap tier list in
swap.tiers =3D=3D "none" or "disabled" Not allowed to swap.

"all", "zswap", "none" are reserved keywords.
"foo", "bar" etc are custom lists of swap tiers. User define custom
tier list in sys/kernel/mm/swap/tiers:
ssd:zswap,/dev/nvme01p4
hdd:/dev/sda4,/dev/sdb4

That would define two custom tiers. "ssd" can use zswap then /dev/nvme01p4.
The exact name of the "swap.tiers" and tiers name are open to suggestions.

>
> The first two are basically what we have for this patch.
> The last one will be added in a future patch.
>
> This is from the userspace perspective. Internally, we can modify
> memcg->writeback to be a pointer or a struct instead of this bool.
> (as you suggested).

Internally I would suggest memcg->swaptiers, the write back name is
somewhat confusing. As your patch indicated. It has two situation:
1. shrinking from zpool to real swapfile. The write back is appropriate her=
e.
 2. zswap store failed (compression ratio too low, out of memory etc).
The write back is confusing here. It is more like writing through or
skip.

>
> This way, the API remains intact and backward compatible
> (and FWIW, I think there are still a lot of values in having simple
> options for the users who have simple memory hierarchies).

swap.tiers can be simple. For example, you can modify your patch to
"swap.tires =3D=3D zswap" to
set zswap.writeback bool to 0 for now. Most of your patch is still re-usabl=
e.

I think we should discuss if we want to keep zswap.writeback in the
future because that would be some code undeletable and functionally
overlap with swap.tiers

Chris