linux-kernel - Re: [RFC] Restrict size of page

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Message-Id: <20101007121816.bbd009c1.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 7 Oct 2010 12:18:16 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	containers@...ts.linux-foundation.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Restrict size of page_cgroup->flags

On Thu, 7 Oct 2010 08:42:04 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-10-07 08:58:58]:
> 
> > On Wed, 6 Oct 2010 19:53:14 +0530
> > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > 
> > > I propose restricting page_cgroup.flags to 16 bits. The patch for the
> > > same is below. Comments?
> > > 
> > > 
> > > Restrict the bits usage in page_cgroup.flags
> > > 
> > > From: Balbir Singh <balbir@...ux.vnet.ibm.com>
> > > 
> > > Restricting the flags helps control growth of the flags unbound.
> > > Restriciting it to 16 bits gives us the possibility of merging
> > > cgroup id with flags (atomicity permitting) and saving a whole
> > > long word in page_cgroup
> > > 
> > > Signed-off-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
> > 
> > Doesn't make sense until you show the usage of existing bits.
> 
> ??
> 
Limiting something for NOT EXISTING PATCH doesn't make sense, in general.


> > And I guess 16bit may be too large on 32bit systems.
> 
> too large on 32 bit systems? My intention is to keep the flags to 16
> bits and then use cgroup id for the rest and see if we can remove
> mem_cgroup pointer
> 

You can't use flags field to store mem_cgroup_id while we use lock bit on it.
We have to store something more stable...as pfn or node-id or zone-id.

It's very racy. 

Thanks,
-Kame

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