linux-kernel - Re: [PATCH] mm: vmscan: ensure reclaiming pages on the lru lists of zone

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Date:	Mon, 23 Jan 2012 12:20:22 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Hillf Danton <dhillf@...il.com>
Cc:	linux-mm@...ck.org, Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ying Han <yinghan@...gle.com>, Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: ensure reclaiming pages on the lru lists of
 zone

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@...il.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.
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