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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Mon, 17 Apr 2023 01:29:03 -0700
Message-ID: <CAJD7tkZFufCacfu-EeqwhQBYXt8dpea1DYhyDgponjFjdLt5Sw@mail.gmail.com>
Subject: Re: [PATCHv4 0/4] zsmalloc: fine-grained fullness and new compaction algorithm
To:     Sergey Senozhatsky <senozhatsky@chromium.org>
Cc:     Yu Zhao <yuzhao@google.com>, Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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Hi Sergey,

On Sun, Apr 16, 2023 at 8:52=E2=80=AFPM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (23/04/16 20:55), Yu Zhao wrote:
> > > Do you run some specific test?
> >
> > E.g.,
> >   tools/testing/selftests/kvm/max_guest_memory_test -c 112 -m 800 -s 80=
0
> > with 112 CPUs and ~770GB DRAM + 32GB zram.
>
> Hmm ...
>
> Something like this maybe?
>
> The src zspage pointer is not NULL-ed after non-empty zspage is
> put back to corresponding fullness list.
>
> ---
>
> @@ -2239,8 +2241,8 @@ static unsigned long __zs_compact(struct zs_pool *p=
ool,
>                 if (fg =3D=3D ZS_INUSE_RATIO_0) {
>                         free_zspage(pool, class, src_zspage);
>                         pages_freed +=3D class->pages_per_zspage;
> -                       src_zspage =3D NULL;
>                 }
> +               src_zspage =3D NULL;
>
>                 if (get_fullness_group(class, dst_zspage) =3D=3D ZS_INUSE=
_RATIO_100
>                     || spin_is_contended(&pool->lock)) {

For my own education, how can this result in the "next is NULL" debug
error Yu Zhao is seeing?

IIUC if we do not set src_zspage to NULL properly after putback, then
we will attempt to putback again after the main loop in some cases.
This can result in a zspage being present more than once in the
per-class fullness list, right?

I am not sure how this can lead to "next is NULL", which sounds like a
corrupted list_head, because the next ptr should never be NULL as far
as I can tell. I feel like I am missing something.