linux-kernel - Re: [patch] mm: memcontrol: support transparent huge pages under pressure

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Date:	Tue, 23 Sep 2014 15:56:55 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Greg Thelen <gthelen@...gle.com>, <linux-mm@...ck.org>,
	Michal Hocko <mhocko@...e.cz>, Dave Hansen <dave@...1.net>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [patch] mm: memcontrol: support transparent huge pages under
 pressure

On Tue, Sep 23, 2014 at 07:48:27AM -0400, Johannes Weiner wrote:
> On Tue, Sep 23, 2014 at 12:29:27PM +0400, Vladimir Davydov wrote:
> > On Mon, Sep 22, 2014 at 10:52:50PM -0700, Greg Thelen wrote:
> > > In this condition, if res usage is at limit then there's no point in
> > > swapping because memsw.usage is already maximal.  Prior to this patch
> > > I think the kernel did the right thing, but not afterwards.
> > > 
> > > Before this patch:
> > >   if res.usage == res.limit, try_charge() indirectly calls
> > >   try_to_free_mem_cgroup_pages(noswap=true)
> > 
> > But this is wrong. If we fail to charge res, we should try to do swap
> > out along with page cache reclaim. Swap out won't affect memsw.usage,
> > but will diminish res.usage so that the allocation may succeed.
> 
> But we know that the memsw limit must be hit as well in that case, and
> swapping only makes progress in the sense that we are then succeeding
> the memory charge.  But we still fail to charge memsw.

Yeah, I admit I said nonsense. The problem Greg pointed out does exist.
I think your second patch (charging memsw before res) should fix it.

Thanks,
Vladimir
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