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Date: Mon, 23 Jan 2012 12:20:22 +0100
From: Johannes Weiner <hannes@cmpxchg.org>
To: Hillf Danton <dhillf@gmail.com>
Cc: linux-mm@kvack.org, Michal Hocko <mhocko@suse.cz>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Ying Han <yinghan@google.com>, Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: vmscan: ensure reclaiming pages on the lru lists of
 zone
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On Mon, Jan 23, 2012 at 12:47:34AM +0800, Hillf Danton wrote:
> Hi all
> 
> For easy review, it is re-prepared based on 3.3-rc1.
> 
> Thanks
> Hillf
> 
> ===cut please===
> From: Hillf Danton <dhillf@gmail.com>
> Subject: [PATCH] mm: vmscan: ensure reclaiming pages on the lru lists of zone
> 
> While iterating over memory cgroup hierarchy, pages are reclaimed from each
> mem cgroup, and reclaim terminates after a full round-trip. It is possible
> that no pages on the lru lists of given zone are reclaimed, as termination
> is checked after the reclaiming function.
> 
> Mem cgroup iteration is rearranged a bit to make sure that pages are reclaimed
> from both mem cgroups and zone.

It's not only possible, it's guaranteed: with the memory controller
enabled, the global per-zone lru lists are empty.

Pages used to be linked on the global per-zone AND the memcg per-zone
lru lists.  Nowadays, they only sit on the memcg per-zone lists, which
is why global reclaim does a hierarchy walk.

The global per-zone lists are just an artifact for when the memory
controller is not available.  The plan is to make root_mem_cgroup
available at all times, even without the memory controller.

So I'm afraid your patch only adds a round of scanning a known-to-be
empty lruvec.  NAK.