linux-kernel - Re: [PATCH] mm: preallocate page before lock_page at filemap COW. (WasRe: [PATCH V2] mm: Do not keep page locked during page fault while charging it for memcg

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Date:	Thu, 23 Jun 2011 15:23:12 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Lutz Vieweg <lvml@....de>
Subject: Re: [PATCH] mm: preallocate page before lock_page at filemap COW.
 (WasRe: [PATCH V2] mm: Do not keep page locked during page fault while
 charging it for memcg

On Thu 23-06-11 22:01:40, Hiroyuki Kamezawa wrote:
> 2011/6/23 Michal Hocko <mhocko@...e.cz>:
> > On Thu 23-06-11 19:01:57, KAMEZAWA Hiroyuki wrote:
> >> On Thu, 23 Jun 2011 11:02:04 +0200
> >> Michal Hocko <mhocko@...e.cz> wrote:
> >>
> >> > On Thu 23-06-11 17:08:11, KAMEZAWA Hiroyuki wrote:
> >> > > On Thu, 23 Jun 2011 09:41:33 +0200
> >> > > Michal Hocko <mhocko@...e.cz> wrote:
> >> > [...]
> >> > > > Other than that:
> >> > > > Reviewed-by: Michal Hocko <mhocko@...e.cz>
> >> > > >
> >> > >
> >> > > I found the page is added to LRU before charging. (In this case,
> >> > > memcg's LRU is ignored.) I'll post a new version with a fix.
> >> >
> >> > Yes, you are right. I have missed that.
> >> > This means that we might race with reclaim which could evict the COWed
> >> > page wich in turn would uncharge that page even though we haven't
> >> > charged it yet.
> >> >
> >> > Can we postpone page_add_new_anon_rmap to the charging path or it would
> >> > just race somewhere else?
> >> >
> >>
> >> I got a different idea. How about this ?
> >> I think this will have benefit for non-memcg users under OOM, too.
> >
> > Could you be more specific? I do not see how preallocation which might
> > turn out to be pointless could help under OOM.
> >
> 
> We'll have no page allocation under lock_page() held in this path.
> I think it is good.

But it can also cause that the page, we are about to fault in, is evicted
due to allocation so we would have to do a major fault... This is
probably not that serious, though.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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