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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>