Date: Wed, 12 Nov 2008 16:46:37 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: balbir@linux.vnet.ibm.com
Cc: kamezawa.hiroyu@jp.fujitsu.com, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, nishimura@mxp.nes.nec.co.jp,
       menage@google.com
Subject: Re: [RFC][PATCH 1/6] memcg: free all at rmdir
Message-Id: <20081112164637.b6f3cb78.akpm@linux-foundation.org>
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On Thu, 13 Nov 2008 05:53:49 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> Andrew Morton wrote:
> > On Wed, 12 Nov 2008 12:26:56 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > 
> >> +5.1 on_rmdir
> >> +set behavior of memcg at rmdir (Removing cgroup) default is "drop".
> >> +
> >> +5.1.1 drop
> >> +       #echo on_rmdir drop > memory.attribute
> >> +       This is default. All pages on the memcg will be freed.
> >> +       If pages are locked or too busy, they will be moved up to the parent.
> >> +       Useful when you want to drop (large) page caches used in this memcg.
> >> +       But some of in-use page cache can be dropped by this.
> >> +
> >> +5.1.2 keep
> >> +       #echo on_rmdir keep > memory.attribute
> >> +       All pages on the memcg will be moved to its parent.
> >> +       Useful when you don't want to drop page caches used in this memcg.
> >> +       You can keep page caches from some library or DB accessed by this
> >> +       memcg on memory.
> > 
> > Would it not be more useful to implement a per-memcg version of
> > /proc/sys/vm/drop_caches?  (One without drop_caches' locking bug,
> > hopefully).
> > 
> > If we do this then we can make the above "keep" behaviour non-optional,
> > and the operator gets to choose whether or not to drop the caches
> > before doing the rmdir.
> > 
> > Plus, we get a new per-memcg drop_caches capability.  And it's a nicer
> > interface, and it doesn't have the obvious races which on_rmdir has,
> > etc.
> > 
> 
> Andrew, I suspect that will not be easy, since we don't track address spaces
> that belong to a particular memcg. If page cache ends up being shared across
> memcg's, dropping them would impact both mem cgroups.
> 

walk the LRUs?
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