linux-kernel - Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

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Date:   Thu, 14 Dec 2023 05:37:15 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     Nhat Pham <nphamcs@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chris Li <chriscli@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

On Thu, Dec 14, 2023 at 5:33 AM Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> On 2023/12/14 08:18, Nhat Pham wrote:
> > On Wed, Dec 13, 2023 at 3:34 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >>
> >> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> >> <zhouchengming@...edance.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.
>
> Right, there is no any history about the reason why we needed 2 pages.
> But obviously only one page is needed from the current code and no any
> problem found in the kernel build stress testing.

Could you try manually stressing the compression with data that
doesn't compress at all (i.e. dlen == PAGE_SIZE)? I want to make sure
that this case is specifically handled. I think using data from
/dev/random will do that but please double check that dlen ==
PAGE_SIZE.