linux-kernel - Re: [patch 2/3] memcg: prevent endless loop on huge page charge

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Message-ID: <20110127141451.GA14512@cmpxchg.org>
Date:	Thu, 27 Jan 2011 15:14:51 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [patch 2/3] memcg: prevent endless loop on huge page charge

Hi Gleb,

On Thu, Jan 27, 2011 at 04:00:24PM +0200, Gleb Natapov wrote:
> On Thu, Jan 27, 2011 at 02:46:45PM +0100, Johannes Weiner wrote:
> > The charging code can encounter a charge size that is bigger than a
> > regular page in two situations: one is a batched charge to fill the
> > per-cpu stocks, the other is a huge page charge.
> > 
> > This code is distributed over two functions, however, and only the
> > outer one is aware of huge pages.  In case the charging fails, the
> > inner function will tell the outer function to retry if the charge
> > size is bigger than regular pages--assuming batched charging is the
> > only case.  And the outer function will retry forever charging a huge
> > page.
> > 
> > This patch makes sure the inner function can distinguish between batch
> > charging and a single huge page charge.  It will only signal another
> > attempt if batch charging failed, and go into regular reclaim when it
> > is called on behalf of a huge page.
> > 
> Yeah, that is exactly the case I am debugging right now. Came up with
> different solution: pass page_size to __mem_cgroup_do_charge() and
> compare csize with page_size (not CHARGE_SIZE). Not sure which solution
> it more correct.

I guess it makes no difference, but using CHARGE_SIZE gets away
without adding another parameter to __mem_cgroup_do_charge().
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