ksm: make rmap walks more scalable

The rmap walks in ksm.c are like those in rmap.c: they can safely be
done with anon_vma_lock_read().

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -1624,7 +1624,7 @@ again:
 		struct anon_vma_chain *vmac;
 		struct vm_area_struct *vma;
 
-		anon_vma_lock_write(anon_vma);
+		anon_vma_lock_read(anon_vma);
 		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
 					       0, ULONG_MAX) {
 			vma = vmac->vma;
@@ -1648,7 +1648,7 @@ again:
 			if (!search_new_forks || !mapcount)
 				break;
 		}
-		anon_vma_unlock(anon_vma);
+		anon_vma_unlock_read(anon_vma);
 		if (!mapcount)
 			goto out;
 	}
@@ -1678,7 +1678,7 @@ again:
 		struct anon_vma_chain *vmac;
 		struct vm_area_struct *vma;
 
-		anon_vma_lock_write(anon_vma);
+		anon_vma_lock_read(anon_vma);
 		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
 					       0, ULONG_MAX) {
 			vma = vmac->vma;
@@ -1697,11 +1697,11 @@ again:
 			ret = try_to_unmap_one(page, vma,
 					rmap_item->address, flags);
 			if (ret != SWAP_AGAIN || !page_mapped(page)) {
-				anon_vma_unlock(anon_vma);
+				anon_vma_unlock_read(anon_vma);
 				goto out;
 			}
 		}
-		anon_vma_unlock(anon_vma);
+		anon_vma_unlock_read(anon_vma);
 	}
 	if (!search_new_forks++)
 		goto again;
@@ -1731,7 +1731,7 @@ again:
 		struct anon_vma_chain *vmac;
 		struct vm_area_struct *vma;
 
-		anon_vma_lock_write(anon_vma);
+		anon_vma_lock_read(anon_vma);
 		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
 					       0, ULONG_MAX) {
 			vma = vmac->vma;
@@ -1749,11 +1749,11 @@ again:
 
 			ret = rmap_one(page, vma, rmap_item->address, arg);
 			if (ret != SWAP_AGAIN) {
-				anon_vma_unlock(anon_vma);
+				anon_vma_unlock_read(anon_vma);
 				goto out;
 			}
 		}
-		anon_vma_unlock(anon_vma);
+		anon_vma_unlock_read(anon_vma);
 	}
 	if (!search_new_forks++)
 		goto again;