On Fri, Jun 10, 2011 at 02:49:35PM -0700, Hugh Dickins wrote:
> On Fri, 10 Jun 2011, KAMEZAWA Hiroyuki wrote:
> >
> > I think this can be a fix.
>
> Sorry, I think not: I've not digested your rationale,
> but three things stand out:
>
> 1. Why has this only just started happening? I may not have run that
> test on 3.0-rc1, but surely I ran it for hours with 2.6.39;
> maybe not with khugepaged, but certainly with ksmd.
>
> 2. Your hunk below:
> > - if (!mm_need_new_owner(mm, p))
> > + if (!mm_need_new_owner(mm, p)) {
> > + rcu_assign_pointer(mm->owner, NULL);
> is now setting mm->owner to NULL at times when we were sure it did not
> need updating before (task is not the owner): you're damaging mm->owner.
>
> 3. There's a patch from Andrea in 3.0-rc1 which looks very likely to be
> relevant, 692e0b35427a "mm: thp: optimize memcg charge in khugepaged".
> I'll try reproducing without that tonight (I crashed in 20 minutes
> this morning, so it's not too hard).
It looks likely. This change moved the memcg charge out of the
mmap_sem read section, which kept the last task of the mm from
exiting:
do_exit
exit_mm
mmput
khugepaged_exit
down_write(&mm->mmap_sem);
up_write(&mm->mmap_sem);
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