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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 2 Nov 2023 13:54:12 -0700
Message-ID: <CAJD7tkacw52fwr6bUk5frepaRN071mmCeGke4s-jMwAXjKqSPg@mail.gmail.com>
Subject: Re: [RFC PATCH v2] zswap: memcontrol: implement zswap writeback disabling
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org,
        tj@kernel.org, lizefan.x@bytedance.com,
        cerasuolodomenico@gmail.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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On Thu, Nov 2, 2023 at 1:50=E2=80=AFPM Johannes Weiner <hannes@cmpxchg.org>=
 wrote:
>
> On Thu, Nov 02, 2023 at 01:27:24PM -0700, Yosry Ahmed wrote:
> > On Thu, Nov 2, 2023 at 1:02=E2=80=AFPM Nhat Pham <nphamcs@gmail.com> wr=
ote:
> > > @@ -201,6 +201,12 @@ int swap_writepage(struct page *page, struct wri=
teback_control *wbc)
> > >                 folio_end_writeback(folio);
> > >                 return 0;
> > >         }
> > > +
> > > +       if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) =
{
> > > +               folio_mark_dirty(folio);
> > > +               return AOP_WRITEPAGE_ACTIVATE;
> > > +       }
> > > +
> >
> > I am not a fan of this, because it will disable using disk swap if
> > "zswap_writeback" is disabled, even if zswap is disabled or the page
> > was never in zswap. The term zswap_writeback makes no sense here tbh.
> >
> > I am still hoping someone else will suggest better semantics, because
> > honestly I can't think of anything. Perhaps something like
> > memory.swap.zswap_only or memory.swap.types which accepts a string
> > (e.g. "zswap"/"all",..).
>
> I had suggested the writeback name.
>
> My thinking was this: from a user pov, the way zswap is presented and
> described, is a fast writeback cache that sits on top of swap. It's
> implemented as this lookaside thing right now, but that's never how it
> was sold. And frankly, that's not how it's expected to work, either.
>
> From the docs:
>
>   Zswap is a lightweight compressed cache for swap pages.
>
>   Zswap evicts pages from compressed cache on an LRU basis to the
>   backing swap device when the compressed pool reaches its size limit.
>
> When zswap is enabled, IO to the swap device is expected to come from
> zswap. Everything else would be a cache failure. There are a few cases
> now where zswap rejects and bypasses to swap. It's not a stretch to
> call them accelerated writeback or writethrough. But also, these
> represent failures and LRU inversions, we're working on fixing them.
>
> So from a user POV it's reasonable to say if I have zswap enabled and
> disable writeback, I don't expect swapfile IO.

Makes sense (although now you had me thinking whether
memory.zswap.writethrough is a better name).

>
> But yes, if zswap isn't enabled at all, this shouldn't prevent pages
> from going to swap.

Right, at least mem_cgroup_zswap_writeback_enabled() should always
return true if zswap is disabled.

One more unrelated thing,  I think we should drop the
cgroup_subsys_on_dfl() check there. I understand the interface is only
exposed in v2, but I don't see any reason why it wouldn't work in v1.
Let's not make it harder for anyone who tries to expose this in v1
(whether upstream or in an internal patch).