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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 5 May 2022 12:30:29 -0700
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Subject: Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage
To:     Johannes Weiner <hannes@cmpxchg.org>,
        Yosry Ahmed <yosryahmed@google.com>,
        Yuanchu Xie <yuanchu@google.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Linux MM <linux-mm@kvack.org>,
        Cgroups <cgroups@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>
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+Yosry & Yuanchu

On Thu, Apr 28, 2022 at 8:17 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
[...]
> >
> > Yes, we have some modifications to zswap to make it work without any
> > backing real swap.
>
> Not sure if you can share them, but I would be interested in those
> changes. We have real backing swap, but because of the way swap
> entries are allocated, pages stored in zswap will consume physical
> disk slots. So on top of regular swap, you need to provision disk
> space for zswap as well, which is unfortunate.
>
> What could be useful is a separate swap entry address space that maps
> zswap slots and disk slots alike. This would fix the above problem. It
> would have the added benefit of making swapoff much simpler and faster
> too, as it doesn't need to chase down page tables to free disk slots.
>

I think we can share the code. Adding Yosry & Yuanchu who are
currently maintaining that piece of code.

Though that code might not be in an upstreamable state. At the high
level, it introduces a new type of swap (SWP_GHOST) which underlying
is a truncated file, so no real disk space is needed. The zswap always
accepts the page, so the kernel never tries to go to the underlying
swapfile (reality is a bit more complicated due to the presence of
incompressible memory and no real disk present on the system).