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


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.

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.

Linus


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