linux-kernel - Re: [PATCH v5] zswap: memcontrol: implement zswap writeback disabling

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Date:   Tue, 21 Nov 2023 19:45:44 -0800
From:   Chris Li <chrisl@...nel.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, tj@...nel.org,
        lizefan.x@...edance.com, Johannes Weiner <hannes@...xchg.org>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Hugh Dickins <hughd@...gle.com>, corbet@....net,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        senozhatsky@...omium.org, rppt@...nel.org,
        linux-mm <linux-mm@...ck.org>, kernel-team@...a.com,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        david@...t.cz, Minchan Kim <minchan@...gle.com>,
        Kairui Song <kasong@...cent.com>,
        Zhongkun He <hezhongkun.hzk@...edance.com>
Subject: Re: [PATCH v5] zswap: memcontrol: implement zswap writeback disabling

On Tue, Nov 21, 2023 at 5:19 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Tue, Nov 21, 2023 at 11:09 AM Chris Li <chrisl@...nel.org> wrote:
> >
> > On Tue, Nov 21, 2023 at 10:13 AM Nhat Pham <nphamcs@...il.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 = no_zswap and memory.zswap.max = 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 version,
> > > (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