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

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

On Thu, May 06, 2010 at 07:15:31AM -0700, Linus Torvalds wrote:
> 
> 
> On Thu, 6 May 2010, Mel Gorman wrote:
> > > 
> > > 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?
> 
> Nothing. And we shouldn't care.
> 
> If it's been freed and re-used, then all the anon_vma's (and vma's) 
> associated with the original anon_vma (and page) have been free'd.
> 
> And that, in turn, means that we don't really need to lock anything at 
> all. The fact that we end up locking an anon_vma that _used_ to be the 
> root anon_vma is immaterial - the lock won't _help_, but it shouldn't hurt 
> either, since it's still a valid spinlock.
> 

I can't see any problem with the logic.

> Now, the above is only true as far as the anon_vma itself is concerned. 
> It's entirely possible that any _other_ data structures would need to be 
> double-checked after getting the lock. For example, is the _page_ still 
> associated with that anon_vma? But that's an external issue as far as the 
> anon_vma locking is concerned - presumably the 'rmap_walk()' caller will 
> have made sure that the page itself is stable somehow.
> 

It does, by having the page locked as it performs the walk.

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

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