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

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Date:   Thu, 14 Dec 2023 10:30:19 -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 5:57 AM Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> On 2023/12/14 21:37, Yosry Ahmed wrote:
> > 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.
>
> I just did the same kernel build testing, indeed there are a few cases
> that output dlen == PAGE_SIZE.
>
> bpftrace -e 'k:zpool_malloc {@[(uint32)arg1==4096]=count()}'
>
> @[1]: 2
> @[0]: 12011430

That's very useful information, thanks for testing that. Please
include this in the commit log. Please also include the fact that we
used to store a zswap header with the compressed page but don't do
that anymore, which *may* be the reason why this was needed back then.

I still want someone who knows the history to Ack this, but FWIW it
looks correct to me, so low-key:
Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>