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Date: Mon, 29 Aug 2011 09:57:31 +0200
From: Johannes Weiner <hannes@cmpxchg.org>
To: Ying Han <yinghan@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        Michal Hocko <mhocko@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
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        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Mel Gorman <mgorman@suse.de>, Greg Thelen <gthelen@google.com>,
        Michel Lespinasse <walken@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>,
        Balbir Singh <balbir@linux.vnet.ibm.com>
Subject: Re: [patch 2/8] mm: memcg-aware global reclaim
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On Mon, Aug 29, 2011 at 12:22:02AM -0700, Ying Han wrote:
> fix hierarchy_walk() to hold a reference to first mem_cgroup
> 
> The first mem_cgroup returned from hierarchy_walk() is used to
> terminate a round-trip. However there is no reference hold on
> that which the first could be removed during the walking. The
> patch including the following change:
> 
> 1. hold a reference on the first mem_cgroup during the walk.
> 2. rename the variable "root" to "target", which we found using
> "root" is confusing in this content with root_mem_cgroup. better
> naming is welcomed.

Thanks for the report.

This was actually not the only case that could lead to overlong (not
necessarily endless) looping.

With several scanning threads, a single thread may not encounter its
first cgroup again for a long time, as the other threads would visit
it.

I changed this to use scan generations.  Restarting the scan from id 0
starts the next scan generation.  The iteration function returns NULL
if the generation changed since a loop was started.

This way, iterators can reliably detect whether they should call it
quits without any requirements for previously encountered memcgs.