[PATCH]mmap: avoid unnecessary anon_vma lock

[Shaohua Li <shaohua.li@intel.com>]

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@intel.com>

---
 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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