linux-kernel - Re: BUG: scheduling while atomic: cron/668/0x10c9a0c0

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Date:	Fri, 3 Jun 2016 11:00:30 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	linux-m68k <linux-m68k@...r.kernel.org>
Subject: Re: BUG: scheduling while atomic: cron/668/0x10c9a0c0

Hi Mel,

On Fri, Jun 3, 2016 at 10:41 AM, Mel Gorman <mgorman@...hsingularity.net> wrote:
> On Fri, Jun 03, 2016 at 09:57:22AM +0200, Geert Uytterhoeven wrote:
>> On Thu, Jun 2, 2016 at 8:43 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
>> > On Thu, 2 Jun 2016 13:19:36 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:
>> >> > >Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
>> >> >
>> >> > Acked-by: Vlastimil Babka <vbabka@...e.cz>
>> >>
>> >> Thanks.
>> >
>> > I queued this.  A tested-by:Geert would be nice?
>> >
>> > From: Mel Gorman <mgorman@...hsingularity.net>
>> > Subject: mm, page_alloc: recalculate the preferred zoneref if the context can ignore memory policies
>> >
>> > The optimistic fast path may use cpuset_current_mems_allowed instead of of
>> > a NULL nodemask supplied by the caller for cpuset allocations.  The
>> > preferred zone is calculated on this basis for statistic purposes and as a
>> > starting point in the zonelist iterator.
>> >
>> > However, if the context can ignore memory policies due to being atomic or
>> > being able to ignore watermarks then the starting point in the zonelist
>> > iterator is no longer correct.  This patch resets the zonelist iterator in
>> > the allocator slowpath if the context can ignore memory policies.  This
>> > will alter the zone used for statistics but only after it is known that it
>> > makes sense for that context.  Resetting it before entering the slowpath
>> > would potentially allow an ALLOC_CPUSET allocation to be accounted for
>> > against the wrong zone.  Note that while nodemask is not explicitly set to
>> > the original nodemask, it would only have been overwritten if
>> > cpuset_enabled() and it was reset before the slowpath was entered.
>> >
>> > Link: http://lkml.kernel.org/r/20160602103936.GU2527@techsingularity.net
>> > Fixes: c33d6c06f60f710 ("mm, page_alloc: avoid looking up the first zone in a zonelist twice")
>>
>> My understanding was that this was an an additional patch, not fixing
>> the problem in-se?
>
> It doesn't fix the problem you had, it is a follow-on patch that
> potentially affects.

Thanks for confirming!

>> Indeed, after applying this patch (without the other one that added
>> "z = ac->preferred_zoneref;" to the reset_fair block of
>> get_page_from_freelist()) I still get crashes...
>
> The patch you have is the only one required for the crash. This patch
> handles a corner case with atomic allocations that can ignore memory
> policies.

OK.

In the mean time my tests completed successfully with both patches applied.

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds