Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeingthe wrong VMA information

[Mel Gorman]

On Wed, May 05, 2010 at 11:02:25AM -0700, Linus Torvalds wrote:
> 
> 
> On Wed, 5 May 2010, Mel Gorman wrote:
> > 
> > If the same_vma list is properly ordered then maybe something like the
> > following is allowed?
> 
> Heh. This is the same logic I just sent out. However:
> 
> > +	anon_vma = page_rmapping(page);
> > +	if (!anon_vma)
> > +		return NULL;
> > +
> > +	spin_lock(&anon_vma->lock);
> 
> RCU should guarantee that this spin_lock() is valid, but:
> 
> > +	/*
> > +	 * Get the oldest anon_vma on the list by depending on the ordering
> > +	 * of the same_vma list setup by __page_set_anon_rmap
> > +	 */
> > +	avc = list_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);
> 
> We're not guaranteed that the 'anon_vma->head' list is non-empty.
> 
> Somebody could have freed the list and the anon_vma and we have a stale 
> 'page->anon_vma' (that has just not been _released_ yet). 
> 

Ahh, fair point. I asked in the other mail was the empty list check
necessary - it is. Thanks

> And shouldn't that be 'list_first_entry'? Or &anon_vma->head.next?
> 

Should have been list_first_entry.

> How did that line actually work for you? Or was it just a "it boots", but 
> no actual testing of the rmap walk?
> 

It's currently running a migration-related stress test without any
deadlocks or lockdep wobbly so far.

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