linux-kernel - Re: [PATCH] z3fold: fix stale list handling

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Message-ID: <CAMJBoFPrf_O4SeE9ve0zo1qaZdocwq=u+mYVAFQTm2NNbx9xqg@mail.gmail.com>
Date:   Fri, 15 Sep 2017 10:34:53 +0200
From:   Vitaly Wool <vitalywool@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Dan Streetman <ddstreet@...e.org>, Oleksiy.Avramchenko@...y.com
Subject: Re: [PATCH] z3fold: fix stale list handling

Hi Andrew,

2017-09-14 23:15 GMT+02:00 Andrew Morton <akpm@...ux-foundation.org>:
> On Thu, 14 Sep 2017 15:59:36 +0200 Vitaly Wool <vitalywool@...il.com> wrote:
>
>> Fix the situation when clear_bit() is called for page->private before
>> the page pointer is actually assigned. While at it, remove work_busy()
>> check because it is costly and does not give 100% guarantee anyway.
>
> Does this fix https://bugzilla.kernel.org/show_bug.cgi?id=196877 ?  If
> so, the bugzilla references and a reported-by should be added.

I wish it did but it doesn't. The bug you are referring to happens
with the "unbuddied" list, and the current version of
z3fold_reclaim_page() just doesn't have that code.
This patch fixes the processing of "stale" lists, with stale lists
having been introduced with the per-CPU unbuddied lists patch, which
is pretty recent.
To fix https://bugzilla.kernel.org/show_bug.cgi?id=196877, we'll have
to either backport per-CPU unbuddied lists plus the two fixes, or
propose a separate fix.

> What are the end-user visible effects of the bug?  Please always
> include this info when fixing bugs.

If page is NULL, clear_bit for page->private will result in a kernel crash.

> Should this fix be backported into -stable kernels?

No, this patch fixes the code that is not in any released kernel yet.

~vitaly