Re: [RFC][PATCH 1/6] memcg: free all at rmdir

[Balbir Singh]

Andrew Morton wrote:
> On Wed, 12 Nov 2008 12:26:56 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> 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.


-- 
	Balbir
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