Date:	Sat, 26 Oct 2013 14:11:48 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Tom Weber <l_linux-kernel@...l2news.4t2.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Automatic NUMA balancing patches for tip-urgent/stable


* Mel Gorman <mgorman@...e.de> wrote:

> On Mon, Oct 07, 2013 at 11:28:38AM +0100, Mel Gorman wrote:
> > This series has roughly the same goals as previous versions despite the
> > size. It reduces overhead of automatic balancing through scan rate reduction
> > and the avoidance of TLB flushes. It selects a preferred node and moves tasks
> > towards their memory as well as moving memory toward their task. It handles
> > shared pages and groups related tasks together. Some problems such as shared
> > page interleaving and properly dealing with processes that are larger than
> > a node are being deferred. This version should be ready for wider testing
> > in -tip.
> > 
> 
> Hi Ingo,
> 
> Off-list we talked with Peter about the fact that automatic NUMA
> balancing as merged in 3.10, 3.11 and 3.12 shortly may corrupt
> userspace memory. There is one LKML report on this that I'm aware of --
> https://lkml.org/lkml/2013/7/31/647 which I prompt forgot to follow up
> properly on . The user-visible effect is that pages get filled with zeros
> with results such as null pointer exceptions in JVMs. It is fairly difficult
> to trigger but it became much easier to trigger during the development of
> the series "Basic scheduler support for automatic NUMA balancing" which
> is how it was discovered and finally fixed.
> 
> In that series I tagged patches 2-9 for -stable as these patches addressed
> the problem for me. I did not call it out as clearly as I should have
> and did not realise the cc: stable tags were stripped. Worse, as it was
> close to the release and the bug is relatively old I was ok with waiting
> until 3.12 came out and then treat it as a -stable backport. It has been
> highlighted that this is the wrong attitude and we should consider merging
> the fixes now and backporting to -stable sooner rather than later.
> 
> The most important patches are 
> 
> mm: Wait for THP migrations to complete during NUMA hinting fault
> mm: Prevent parallel splits during THP migration
> mm: Close races between THP migration and PMD numa clearing
> 
> but on their own they will cause conflicts with tricky fixups and -stable
> would differ from mainline in annoying ways. Patches 2-9 have been heavily
> tested in isolation so I'm reasonably confident they fix the problem and are
> -stable material. While strictly speaking not all the patches are required
> for the fix, the -stable kernels would then be directly comparable with
> 3.13 when the full NUMA balancing series is applied. If I rework them at
> this point then I'll also have to retest delaying things until next week.
> 
> Please consider queueing patches 2-9 for 3.12 via -urgent if it is 
> not too late and preserve the cc: stable tags so Greg will pick 
> them up automatically.

Would be nice if you gave me all the specific SHA1 tags of 
sched/core that are required for the fix. We can certainly
use a range to make it all safer to apply.

Thanks,

	Ingo
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