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

[Mel Gorman <mel@csn.ul.ie>]

On Wed, May 05, 2010 at 05:42:19PM -0700, Linus Torvalds wrote:
> 
> 
> On Thu, 6 May 2010, Mel Gorman wrote:
> > +	/*
> > +	 * Get the root anon_vma on the list by depending on the ordering
> > +	 * of the same_vma list setup by __page_set_anon_rmap. Basically
> > +	 * we are doing
> > +	 *
> > +	 * local anon_vma -> local vma -> deepest vma -> anon_vma
> > +	 */
> > +	avc = list_first_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);
> > +	vma = avc->vma;
> > +	root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
> > +	root_anon_vma = root_avc->anon_vma;
> > +	if (!root_anon_vma) {
> > +		/* XXX: Can this happen? Don't think so but get confirmation */
> > +		WARN_ON_ONCE(1);
> > +		return anon_vma;
> > +	}
> 
> No, that can't happen. If you find an avc struct, it _will_ have a 
> anon_vma pointer. So there's no point in testing for NULL. If some bug 
> happens, you're much better off with the oops than with the warning.
> 

Good. If this returns NULL, it should oops when spin_lock(NULL->lock)
is called.

> > +	/* Get the lock of the root anon_vma */
> > +	if (anon_vma != root_anon_vma) {
> > +		/*
> > +		 * XXX: This doesn't seem safe. What prevents root_anon_vma
> > +		 * getting freed from underneath us? Not much but if
> > +		 * we take the second lock first, there is a deadlock
> > +		 * possibility if there are multiple callers of rmap_walk
> > +		 */
> > +		spin_unlock(&anon_vma->lock);
> > +		spin_lock(&root_anon_vma->lock);
> > +	}
> 
> What makes this ok is the fact that it must be running under the RCU read 
> lock, and anon_vma's thus cannot be released.

This is very subtle in itself. RCU guarantees that the anon_vma exists
but does it guarantee that it's the same one we expect and that it
hasn't been freed and reused?

> My version of the code made 
> that explicit. Yours does not, and doesn't even have comments about the 
> fact that it needs to be called RCU read-locked. Tssk, tssk.
> 

I added a comment.

> Please don't just assume locking. Either lock it, or say "this must be 
> called with so-and-so held". Not just a silent "this would be buggy if 
> anybody ever called it without the RCU lock".
> 

Sure. It was an oversight when merging what I had with what you posted
up.

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

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