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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Message-ID: <20110623115855.GF31593@tiehlicka.suse.cz>
Date:	Thu, 23 Jun 2011 13:58:55 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	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 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.

> 
> A concerns is VM_FAULT_RETRY case but wait-for-lock will be much heavier
> than preallocation + free-for-retry cost.

Preallocation is rather costly when fault handler fails (e.g. SIGBUS
which is the easiest one to trigger).

I am not saying this approach is bad but I think that preallocation can
be much more costly than unlock, charge and lock&recheck approach.
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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