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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Wed, 13 Dec 2023 16:18:28 -0800
Message-ID: <CAKEwX=P4=YbvoRCa5+BB+==f4YL_5-6AaUNUCdH3v2faTx-PYQ@mail.gmail.com>
Subject: Re: [PATCH 2/5] mm/zswap: change dstmem size to one page
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Chengming Zhou <zhouchengming@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chris Li <chriscli@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Wed, Dec 13, 2023 at 3:34=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Tue, Dec 12, 2023 at 8:18=E2=80=AFPM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
> >
> > Change the dstmem size from 2 * PAGE_SIZE to only one page since
> > we only need at most one page when compress, and the "dlen" is also
> > PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
> > we don't wanna store the output in zswap anyway.
> >
> > So change it to one page, and delete the stale comment.
>
> I couldn't find the history of why we needed 2 * PAGE_SIZE, it would
> be nice if someone has the context, perhaps one of the maintainers.

It'd be very nice indeed.

>
> One potential reason is that we used to store a zswap header
> containing the swap entry in the compressed page for writeback
> purposes, but we don't do that anymore. Maybe we wanted to be able to
> handle the case where an incompressible page would exceed PAGE_SIZE
> because of that?

It could be hmm. I didn't study the old zswap architecture too much,
but it has been 2 * PAGE_SIZE since the time zswap was first merged
last I checked.
I'm not 100% comfortable ACK-ing the undoing of something that looks
so intentional, but FTR, AFAICT, this looks correct to me.