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From:   Chris Li <chrisl@kernel.org>
Date:   Tue, 21 Nov 2023 19:45:44 -0800
Message-ID: <CAF8kJuNbgsMK-JiBvj=O6UbueNNeo5WJnJdwrH7JAuavNHH6Lg@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>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <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, Minchan Kim <minchan@google.com>,
        Kairui Song <kasong@tencent.com>,
        Zhongkun He <hezhongkun.hzk@bytedance.com>
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On Tue, Nov 21, 2023 at 5:19=E2=80=AFPM Nhat Pham <nphamcs@gmail.com> wrote=
:
>
> On Tue, Nov 21, 2023 at 11:09=E2=80=AFAM Chris Li <chrisl@kernel.org> wro=
te:
> >
> > On Tue, Nov 21, 2023 at 10:13=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> =
wrote:
> > >
> > > Hi Chris!
> > >
> > > Thanks for the patch. Would you mind if I spend some time staring
> > > at the suggestion again and testing it some more?
> >
> > Of course, by all means. That is just the minimal version to be
> > functional compatible with your zswap.writeback.
> >
> > I might consider a follow up patch to add "no_zswap" and "none" to
> > convert the SSD only swapfile, which can't be expressed by
> > zswap.writeback.
> > That should cover all 4 combinations of zswap and swap files without
> > creating a custom swap tiers list.
> >
> > "all": zswap + swapfile
> > "zswap": zswap only
> > "no_zswap": swapfile only.
> > "none": no swap.
> >
> > All keyword names are open to suggestions.
>
> SGTM! There might be some functionality duplication between
> memory.swap.tiers =3D no_zswap and memory.zswap.max =3D 0, but
> otherwise this seems reasonable to me.

Yes, there is some function duplication. However, there is some small
difference that no_zswap will not enter zswap code at all. Vs
memory.zswap.max will take a short trip into zswap code to find out
Oops, not zswap for you.

>
> no_zswap sounds a bit awkward, but I can't come up with a better
> name.

Again, I am open to better suggestions.
I have also considered "!zswap", "!" has special meaning in bash, so
it will require quoting in bash.
How about "-zswap"? This does not require special quoting in bash.

>
> >
> > >
> > > If everything is good, I'll squash this patch with the original versi=
on,
> > > (keeping you as a co-developer of the final patch of course), and
> > > update the documentation before re-sending everything as v6.
> >
> > Great!
> >
> > >
> > > Anyway, have a nice Thanksgiving break everyone! Thanks for
> > > taking the time to review my patch and discuss the API with me!
> >
> > My pleasure to discuss the swap with you. We should do the online
> > "swap meet" and invite other developers who are interested in the swap
> > area as well.
>
> I look forward to this meeting! I'd love to discuss more about (z)swap
> development (and more generally, multi-tier memory management).

Let me arrange one then. I am thinking maybe every second week of the
month. That can avoid thanksgiving, christmas and new year.

Let me throw in some more ideas: writing compressed zswap data to SSD
without swap cache.

> Generic page promoter/demoter that takes into account workload
> (cgroup), access recency (LRU + generations)/frequency, and tier
> characteristics (latency, bandwidth, etc.) will be awesome to explore!

Sounds great. Looking forward to it.

Chris