mmap: avoid merging cloned VMAs

Avoid merging a VMA with another VMA which is cloned from the parent process.

The cloned VMA shares the anon_vma lock with the parent process's VMA.  If
we do the merge, more vmas (even the new range is only for current
process) use the perent process's anon_vma lock.  This introduces
scalability issues.  find_mergeable_anon_vma() already considers this
case.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: 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

@@ -703,9 +703,17 @@ static inline int is_mergeable_vma(struct vm_area_struct *vma,
 }
 
 static inline int is_mergeable_anon_vma(struct anon_vma *anon_vma1,
-					struct anon_vma *anon_vma2)
+					struct anon_vma *anon_vma2,
+					struct vm_area_struct *vma)
 {
-	return !anon_vma1 || !anon_vma2 || (anon_vma1 == anon_vma2);
+	/*
+	 * The list_is_singular() test is to avoid merging VMA cloned from
+	 * parents. This can improve scalability caused by anon_vma lock.
+	 */
+	if ((!anon_vma1 || !anon_vma2) && (!vma ||
+		list_is_singular(&vma->anon_vma_chain)))
+		return 1;
+	return anon_vma1 == anon_vma2;
 }
 
 /*
@@ -724,7 +732,7 @@ can_vma_merge_before(struct vm_area_struct *vma, unsigned long vm_flags,
 	struct anon_vma *anon_vma, struct file *file, pgoff_t vm_pgoff)
 {
 	if (is_mergeable_vma(vma, file, vm_flags) &&
-	    is_mergeable_anon_vma(anon_vma, vma->anon_vma)) {
+	    is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
 		if (vma->vm_pgoff == vm_pgoff)
 			return 1;
 	}
@@ -743,7 +751,7 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
 	struct anon_vma *anon_vma, struct file *file, pgoff_t vm_pgoff)
 {
 	if (is_mergeable_vma(vma, file, vm_flags) &&
-	    is_mergeable_anon_vma(anon_vma, vma->anon_vma)) {
+	    is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
 		pgoff_t vm_pglen;
 		vm_pglen = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
 		if (vma->vm_pgoff + vm_pglen == vm_pgoff)
@@ -821,7 +829,7 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 				can_vma_merge_before(next, vm_flags,
 					anon_vma, file, pgoff+pglen) &&
 				is_mergeable_anon_vma(prev->anon_vma,
-						      next->anon_vma)) {
+						      next->anon_vma, NULL)) {
 							/* cases 1, 6 */
 			err = vma_adjust(prev, prev->vm_start,
 				next->vm_end, prev->vm_pgoff, NULL);