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From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
Date:   Wed, 21 Jun 2023 16:03:52 +0900
Message-ID: <CAB=+i9TyX11=30yNiEq5UKfLJ9aBRuJEF6yQAQAnybhzkReLSg@mail.gmail.com>
Subject: Re: [BUG mm-unstable] "kernel BUG at mm/swap.c:393!" on commit b9c91c43412f2e
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Nhat Pham <nphamcs@gmail.com>,
        Domenico Cerasuolo <cerasuolodomenico@gmail.com>,
        Yu Zhao <yuzhao@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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On Wed, Jun 21, 2023 at 4:01=E2=80=AFPM Hyeonggon Yoo <42.hyeyoo@gmail.com>=
 wrote:
>
> On Wed, Jun 07, 2023 at 07:51:43PM +0000, Yosry Ahmed wrote:
> > Commit 71024cb4a0bf ("frontswap: remove frontswap_tmem_exclusive_gets")
> > removed support for exclusive loads from frontswap as it was not used.
> > Bring back exclusive loads support to frontswap by adding an "exclusive=
"
> > output parameter to frontswap_ops->load.
> >
> > On the zswap side, add a module parameter to enable/disable exclusive
> > loads, and a config option to control the boot default value.
> > Refactor zswap entry invalidation in zswap_frontswap_invalidate_page()
> > into zswap_invalidate_entry() to reuse it in zswap_frontswap_load() if
> > exclusive loads are enabled.
> >
> > With exclusive loads, we avoid having two copies of the same page in
> > memory (compressed & uncompressed) after faulting it in from zswap. On
> > the other hand, if the page is to be reclaimed again without being
> > dirtied, it will be re-compressed. Compression is not usually slow, and
> > a page that was just faulted in is less likely to be reclaimed again
> > soon.
> >
> > Suggested-by: Yu Zhao <yuzhao@google.com>
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >
> > v1 -> v2:
> > - Add a module parameter to control whether exclusive loads are enabled
> >   or not, the config option now controls the default boot value instead=
.
> >   Replaced frontswap_ops->exclusive_loads by an output parameter to
> >   frontswap_ops->load() (Johannes Weiner).
> > ---
>
> Hi Yosry, I was testing the latest mm-unstable and encountered a bug.
> It was bisectable and this is the first bad commit.

Oh, I forgot to mention how I was able to reproduce this:

$ stress-ng --bigheap 12
(my box have 24 cores)

>
> Attached config file and bisect log.
> The oops message is available at:
>
> https://social.kernel.org/media/eace06d71655b3cc76411366573e4a8ce240ad65b=
8fd20977d7c73eec9dc2253.jpg
>
> (the head commit is b9c91c43412f2e07 "mm: zswap: support exclusive loads"=
)
> (it's an image because I tested it on real machine)
>
>
> This is what I have as swap space:
>
> $ cat /proc/swaps
> Filename                                Type            Size            U=
sed            Priority
> /var/swap                               file            134217724       0=
               -2
> /dev/zram0                              partition       8388604         0=
               100