[RFC] Restrict size of page_cgroup->flags

KAMEZAWA Hiroyuki kamezawa.hiroyu at jp.fujitsu.com
Wed Oct 6 20:18:16 PDT 2010


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

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> [2010-10-07 08:58:58]:
> 
> > On Wed, 6 Oct 2010 19:53:14 +0530
> > Balbir Singh <balbir at linux.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 at linux.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 at linux.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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