linux-kernel - Re: [PATCH v3 2/2] mm/page_alloc: fix memmap_init

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Date:   Sat, 3 Mar 2018 02:08:27 +0100
From:   Daniel Vacek <neelx@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Paul Burton <paul.burton@...tec.com>, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/2] mm/page_alloc: fix memmap_init_zone pageblock alignment

On Sat, Mar 3, 2018 at 1:40 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Sat,  3 Mar 2018 01:12:26 +0100 Daniel Vacek <neelx@...hat.com> wrote:
>
>> Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
>> where possible") introduced a bug where move_freepages() triggers a
>> VM_BUG_ON() on uninitialized page structure due to pageblock alignment.
>
> b92df1de5d28 was merged a year ago.  Can you suggest why this hasn't
> been reported before now?

Yeah. I was surprised myself I couldn't find a fix to backport to
RHEL. But actually customers started to report this as soon as 7.4
(where b92df1de5d28 was merged in RHEL) was released. I remember
reports from September/October-ish times. It's not easily reproduced
and happens on a handful of machines only. I guess that's why. But
that does not make it less serious, I think.

Though there actually is a report here:
https://bugzilla.kernel.org/show_bug.cgi?id=196443

And there are reports for Fedora from July:
https://bugzilla.redhat.com/show_bug.cgi?id=1473242
and CentOS: https://bugs.centos.org/view.php?id=13964
and we internally track several dozens reports for RHEL bug
https://bugzilla.redhat.com/show_bug.cgi?id=1525121

Enough? ;-)

> This makes me wonder whether a -stable backport is really needed...

For some machines it definitely is. Won't hurt either, IMHO.

--nX