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

[Mel Gorman]

On Thu, Mar 25, 2010 at 06:09:34PM +0900, KOSAKI Motohiro wrote:
> > On Thu, 25 Mar 2010 08:32:35 +0000
> > Mel Gorman <mel@xxxxxxxxx> 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.
> 
> But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
> is meaningless now.

Christoph is opposed to removing it because of cache-hotness issues more
so than use-after-free concerns. The refcount is needed with or without
SLAB_DESTROY_BY_RCU.

> I agree you if we will remove SLAB_DESTROY_BY_RCU
> in the future.
> 
> refcount is easy understanding than rcu trick.
> 
> 
> >  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...)
> 
> agreed.
> 
> > 
> > Thanks,
> > -Kame
> > 
> > 
> 
> 
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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