Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 25 Mar 2010 08:32:35 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> On Thu, Mar 25, 2010 at 11:49:23AM +0900, KOSAKI Motohiro wrote:
> > > On Fri, Mar 19, 2010 at 03:21:41PM +0900, KOSAKI Motohiro wrote: 
> > Hmmm...
> > I haven't understand your mention because I guess I was wrong.
> > 
> > probably my last question was unclear. I mean,
> > 
> > 1) If we still need SLAB_DESTROY_BY_RCU, why do we need to add refcount?
> >     Which difference is exist between normal page migration and compaction?
> 
> The processes typically calling migration today own the page they are moving
> and is not going to exit unexpectedly during migration.
> 
> > 2) If we added refcount, which race will solve?
> > 
> 
> The process exiting and the last anon_vma being dropped while compaction
> is running. This can be reliably triggered with compaction.
> 
> > IOW, Is this patch fix old issue or compaction specific issue?
> > 
> 
> Strictly speaking, it's an old issue but in practice it's impossible to
> trigger because the process migrating always owns the page. Compaction
> moves pages belonging to arbitrary processes.
> 
Kosaki-san,

 IIUC, the race in memory-hotunplug was fixed by this patch [2/11].

 But, this behavior of unmap_and_move() requires access to _freed_
 objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
 it't not good habit in general.

 After direct compaction, page-migration will be one of "core" code of
 memory management. Then, I agree to patch [1/11] as our direction for
 keeping sanity and showing direction to more updates. Maybe adding
 refcnt and removing RCU in futuer is good.

 IMHO, pushing this patch [2/11] as "BUGFIX" independent of this set and
 adding anon_vma->refcnt [1/11] and [3/11] in 1st Direct-compaction patch
 series  to show the direction will makse sense.
 (I think merging 1/11 and 3/11 will be okay...)

Thanks,
-Kame