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

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Dec 07, 2011 at 07:24:59PM -0800, David Rientjes wrote:
> 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.

Yes it's needed, either this or the anon_vma lock around
move_page_tables. Then we also need the i_mmap_mutex around fork or a
triple loop in vmtruncate (then we could remove i_mmap_mutex in
mremap).

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