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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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:
> > > > > > then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount.
> > > > > > So, I think we don't need your [1/11] patch.
> > > > > > 
> > > > > > Am I missing something?
> > > > > > 
> > > > > 
> > > > > The refcount is still needed. The anon_vma might be valid, but the
> > > > > refcount is what ensures that the anon_vma is not freed and reused.
> > > > 
> > > > please please why do we need both mechanism. now cristoph is very busy and I am
> > > > de fact reviewer of page migration and mempolicy code. I really hope to understand
> > > > your patch.
> > > 
> > > As in, why not drop the RCU protection of anon_vma altogeter? Mainly, because I
> > > think it would be reaching too far for this patchset and it should be done as
> > > a follow-up. Putting the ref-count everywhere will change the cache-behaviour
> > > of anon_vma more than I'd like to slip into a patchset like this. Secondly,
> > > Christoph mentions that SLAB_DESTROY_BY_RCU is used to keep anon_vma cache-hot.
> > > For these reasons, removing RCU from these paths and adding the refcount
> > > in others is a patch that should stand on its own.
> > 
> > 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.

Do you mean current memroy hotplug code is broken???
I think compaction need refcount, hotplug also need it. both they migrate another
task's page.

but , I haven't seen hotplug failure. Am I  missing something? or the compaction
have its specific race situation?