Re: [PATCH]mmap: avoid unnecessary anon_vma lock

[Hugh Dickins]

On Mon, 28 Mar 2011, Shaohua Li wrote:

> If we only change vma->vm_end, we can avoid taking anon_vma lock even 'insert'
> isn't NULL, which is the case of split_vma.
> From my understanding, we need the lock before because rmap must get the
> 'insert' VMA when we adjust old VMA's vm_end (the 'insert' VMA is linked to
> anon_vma list in __insert_vm_struct before).
> But now this isn't true any more. The 'insert' VMA is already linked to
> anon_vma list in __split_vma(with anon_vma_clone()) instead of
> __insert_vm_struct. There is no race rmap can't get required VMAs.
> So the anon_vma lock is unnecessary, and this can reduce one locking in brk
> case and improve scalability.
> 
> Signed-off-by: Shaohua Li<shaohua.li@xxxxxxxxx>

I was sceptical at first, but yes, you're right: thanks.
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

> 
> ---
>  mm/mmap.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux/mm/mmap.c
> ===================================================================
> --- linux.orig/mm/mmap.c	2011-03-24 09:08:27.000000000 +0800
> +++ linux/mm/mmap.c	2011-03-24 09:14:03.000000000 +0800
> @@ -605,7 +605,7 @@ again:			remove_next = 1 + (end > next->
>  	 * lock may be shared between many sibling processes.  Skipping
>  	 * the lock for brk adjustments makes a difference sometimes.
>  	 */
> -	if (vma->anon_vma && (insert || importer || start != vma->vm_start)) {
> +	if (vma->anon_vma && (importer || start != vma->vm_start)) {
>  		anon_vma = vma->anon_vma;
>  		anon_vma_lock(anon_vma);
>  	}
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