linux-kernel - Re: [PATCH 2/3] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

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Date:	Wed, 28 Apr 2010 01:10:07 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On Tue, Apr 27, 2010 at 10:30:51PM +0100, Mel Gorman wrote:
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f90ea92..61d6f1d 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -578,6 +578,9 @@ again:			remove_next = 1 + (end > next->vm_end);
>  		}
>  	}
>  
> +	if (vma->anon_vma)
> +		spin_lock(&vma->anon_vma->lock);
> +
>  	if (root) {
>  		flush_dcache_mmap_lock(mapping);
>  		vma_prio_tree_remove(vma, root);
> @@ -620,6 +623,9 @@ again:			remove_next = 1 + (end > next->vm_end);
>  	if (mapping)
>  		spin_unlock(&mapping->i_mmap_lock);
>  
> +	if (vma->anon_vma)
> +		spin_unlock(&vma->anon_vma->lock);
> +
>  	if (remove_next) {
>  		if (file) {
>  			fput(file);

The old code did:

    /*
     * When changing only vma->vm_end, we don't really need
     * anon_vma lock.
     */
    if (vma->anon_vma && (insert || importer || start !=  vma->vm_start))
	anon_vma = vma->anon_vma;
    if (anon_vma) {
        spin_lock(&anon_vma->lock);

why did it become unconditional? (and no idea why it was removed)

But I'm not sure about this part.... this is really only a question, I
may well be wrong, I just don't get it.
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