Re: [PATCH] mremap: enforce rmap src/dst vma ordering in case of vma_merge succeeding in copy_vma

[David Rientjes <rientjes@google.com>]

On Sat, 5 Nov 2011, Andrea Arcangeli wrote:

> migrate was doing a rmap_walk with speculative lock-less access on
> pagetables. That could lead it to not serialize properly against
> mremap PT locks. But a second problem remains in the order of vmas in
> the same_anon_vma list used by the rmap_walk.
> 
> If vma_merge would succeed in copy_vma, the src vma could be placed
> after the dst vma in the same_anon_vma list. That could still lead
> migrate to miss some pte.
> 
> This patch adds a anon_vma_moveto_tail() function to force the dst vma
> at the end of the list before mremap starts to solve the problem.
> 
> If the mremap is very large and there are a lots of parents or childs
> sharing the anon_vma root lock, this should still scale better than
> taking the anon_vma root lock around every pte copy practically for
> the whole duration of mremap.
> 
> Update: Hugh noticed special care is needed in the error path where
> move_page_tables goes in the reverse direction, a second
> anon_vma_moveto_tail() call is needed in the error path.
> 

Is this still needed?  It's missing in linux-next.

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