linux-kernel - Re: [RFC PATCH -v3] take all anon_vma locks in anon_vma

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Date:	Thu, 29 Apr 2010 09:44:52 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH -v3] take all anon_vma locks in anon_vma_lock

On Thu, Apr 29, 2010 at 05:32:17PM +0900, Minchan Kim wrote:
> On Thu, 2010-04-29 at 09:15 +0100, Mel Gorman wrote:
> > On Wed, Apr 28, 2010 at 04:57:34PM -0400, Rik van Riel wrote:
> > > Take all the locks for all the anon_vmas in anon_vma_lock, this properly
> > > excludes migration and the transparent hugepage code from VMA changes done
> > > by mmap/munmap/mprotect/expand_stack/etc...
> > > 
> > 
> > In vma_adjust(), what prevents something like rmap_map seeing partial
> > updates while the following lines execute?
> > 
> >         vma->vm_start = start;
> >         vma->vm_end = end;
> >         vma->vm_pgoff = pgoff;
> >         if (adjust_next) {
> >                 next->vm_start += adjust_next << PAGE_SHIFT;
> >                 next->vm_pgoff += adjust_next;
> >         }
> > They would appear to happen outside the lock, even with this patch. The
> > update happened within the lock in 2.6.33.
> > 
> > 
> > 
> This part does it. :)
> 
> ----
> @@ -578,6 +578,7 @@ again:                      remove_next = 1 + (end >
> next->vm_end);
>                 }   
>         }   
>  
> +       anon_vma_lock(vma, &mm->mmap_sem);
>         if (root) {
>                 flush_dcache_mmap_lock(mapping);
>                 vma_prio_tree_remove(vma, root);
> @@ -599,6 +600,7 @@ again:                      remove_next = 1 + (end >
> next->vm_end);
>                 vma_prio_tree_insert(vma, root);
>                 flush_dcache_mmap_unlock(mapping);
>         }   
> +       anon_vma_unlock(vma);
> ---
> 

I'm blind. You're right.

> But we still need patch about shift_arg_pages.
> 

Assuming you are referring to migration, it's easiest to just not migrate
pages within the stack until after shift_arg_pages runs. The locks
cannot be held during move_page_tables() because the page allocator is
called. It could be done in two stages where pages are allocated outside
the lock and then passed to move_page_tables() but I don't think
increasing the cost of exec() is justified just so a page can be
migrated during exec.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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