Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmasof a mergeable VMA

[Linus Torvalds]

On Sat, 10 Apr 2010, Borislav Petkov wrote:
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date: Sat, Apr 10, 2010 at 01:12:46PM -0700
> 
> > So I'm actually pretty optimistic that this really is it.
> 
> Ok, let me verify what/in which order should be tested before I test
> something wrongly. The RCU-safe fix for the TLB flush can stay for
> correctness reasons, this last patch, obviosly, what happens with the
> find_mergeable_anon_vma() changes to use only singleton lists for
> merging? Should I keep those too?

Yes. So the patches I actually think are important are:

 - the RCU fix is real, although admittedly the race window is probably 
   too small to ever really hit.

 - the simplification rule to find_mergeable_anon_vma's is required, 
   because otherwise our anon_vma_merge() will do the wrong thing (maybe 
   Johannes' patch would be an alternative, but quite frankly, I think we 
   want the simpler code, and I don't think we even _want_ to share 
   anon_vma's that are complex due to forking)

   I like my "cleanup" version (the bigger one with lots of comments) more 
   than the two-liner version, but they should be equivalent.

 - the vma_adjust() fix is the one that I think may actually end up fixing 
   your problems for good. Knock wood.

So I think they are all required, but I suspect that the vma_adjust() one 
is finally the most direct explanation of the problem you've seen.

		Linus
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