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SubjectRe: [rfc patch] mm, oom: fix unnecessary killing of additional processes
On Wed 13-06-18 22:20:49, Tetsuo Handa wrote:
> On 2018/06/05 17:57, Michal Hocko wrote:
> >> For this reason, we see testing harnesses often oom killed immediately
> >> after running a unittest that stresses reclaim or compaction by inducing a
> >> system-wide oom condition. The harness spawns the unittest which spawns
> >> an antagonist memory hog that is intended to be oom killed. When memory
> >> is mlocked or there are a large number of threads faulting memory for the
> >> antagonist, the unittest and the harness itself get oom killed because the
> >> oom reaper sets MMF_OOM_SKIP; this ends up happening a lot on powerpc.
> >> The memory hog has mm->mmap_sem readers queued ahead of a writer that is
> >> doing mmap() so the oom reaper can't grab the sem quickly enough.
> >
> > How come the writer doesn't back off. mmap paths should be taking an
> > exclusive mmap sem in killable sleep so it should back off. Or is the
> > holder of the lock deep inside mmap path doing something else and not
> > backing out with the exclusive lock held?
> >
>
> Here is an example where the writer doesn't back off.
>
> http://lkml.kernel.org/r/20180607150546.1c7db21f70221008e14b8bb8@linux-foundation.org
>
> down_write_killable(&mm->mmap_sem) is nothing but increasing the possibility of
> successfully back off. There is no guarantee that the owner of that exclusive
> mmap sem will not be blocked by other unkillable waits.

but we are talking about mmap() path here. Sure there are other paths
which might need a back off while the lock is held and that should be
addressed if possible but this is not really related to what David wrote
above and I tried to understand.

--
Michal Hocko
SUSE Labs

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