Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

[Johannes Weiner <hannes@cmpxchg.org>]

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);