linux-kernel - Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

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Date:   Thu, 5 May 2022 16:54:02 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Suleiman Souhlal <suleiman@...gle.com>,
        Yang Shi <shy828301@...il.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

On Thu, May 5, 2022 at 3:25 PM Suleiman Souhlal <suleiman@...gle.com> wrote:
>
> On Fri, May 6, 2022 at 4:33 AM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > On Thu, Apr 28, 2022 at 9:54 AM Yang Shi <shy828301@...il.com> wrote:
> > >
> > [...]
> > > > Yes, we have some modifications to zswap to make it work without any
> > > > backing real swap. Though there is a future plan to move to zram
> > > > eventually.
> > >
> > > Interesting, if so why not just simply use zram?
> > >
> >
> > Historical reasons. When we started trying out the zswap, I think zram
> > was still in staging or not stable enough (Suleiman can give a better
> > answer).
>
> One of the reasons we chose zswap instead of zram is that zswap can
> reject pages.
> Also, we wanted to have per-memcg pools, which zswap made much easier to do.

Yes, it was a design choice. zswap was cache-like (tiering) and zram
was storage-like (endpoint). Though nowadays the distinction is
blurry.

It had nothing to do with zram being in staging -- when we took zswap,
it was out of the tree.