mmap: avoid unnecessary anon_vma lock

If we only change vma->vm_end, we can avoid taking anon_vma lock even if
'insert' isn't NULL, which is the case of split_vma.

As I understand it, 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>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -609,7 +609,7 @@ again:			remove_next = 1 + (end > next->vm_end);
 	 * 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);
 	}