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    SubjectRe: Automatic NUMA balancing patches for tip-urgent/stable

    * Mel Gorman <mgorman@suse.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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