linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

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Date:	Sat, 10 Apr 2010 13:40:39 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
cc:	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA



On Sat, 10 Apr 2010, Borislav Petkov wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> 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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