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

[Mel Gorman]

On Thu, Mar 18, 2010 at 09:48:08AM +0900, KOSAKI Motohiro wrote:
> > > > +		/*
> > > > +		 * If the page has no mappings any more, just bail. An
> > > > +		 * unmapped anon page is likely to be freed soon but worse,
> > > > +		 * it's possible its anon_vma disappeared between when
> > > > +		 * the page was isolated and when we reached here while
> > > > +		 * the RCU lock was not held
> > > > +		 */
> > > > +		if (!page_mapcount(page)) {
> > > > +			rcu_read_unlock();
> > > > +			goto uncharge;
> > > > +		}
> > > 
> > > I haven't understand what prevent this check. Why don't we need following scenario?
> > > 
> > >  1. Page isolated for migration
> > >  2. Passed this if (!page_mapcount(page)) check
> > >  3. Process exits
> > >  4. page_mapcount(page) drops to zero so anon_vma was no longer reliable
> > > 
> > > Traditionally, page migration logic is, it can touch garbarge of anon_vma, but
> > > SLAB_DESTROY_BY_RCU prevent any disaster. Is this broken concept?
> > 
> > The check is made within the RCU read lock. If the count is positive at
> > that point but goes to zero due to a process exiting, the anon_vma will
> > still be valid until rcu_read_unlock() is called.
> 
> Thank you!
> 
> 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.

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