[PATCH -mm] do_migrate_pages() calls migrate_to_node() even if task is already on a correct node

[Larry Woodman <lwoodman@xxxxxxxxxx>]

While moving tasks between cpusets I noticed some strange behavior.  Specifically if the nodes of the destination
cpuset are a subset of the nodes of the source cpuset do_migrate_pages() will move pages that are already on a node
in the destination cpuset.  The reason for this is do_migrate_pages() does not check whether each node in the source
nodemask is in the destination nodemask before calling migrate_to_node().  If we simply do this check and skip them
when the source is in the destination moving we wont move nodes that dont need to be moved.
 
Adding a little debug printk to migrate_to_node():

Without this change migrating tasks from a cpuset containing nodes 0-7 to a cpuset containing nodes 3-4, we migrate
from ALL the nodes even if they are in the both the source and destination nodesets:

  Migrating 7 to 4
  Migrating 6 to 3
  Migrating 5 to 4
  Migrating 4 to 3
  Migrating 1 to 4
  Migrating 3 to 4
  Migrating 0 to 3
  Migrating 2 to 3


With this change we only migrate from nodes that are not in the destination nodesets:

  Migrating 7 to 4
  Migrating 6 to 3
  Migrating 5 to 4
  Migrating 2 to 3
  Migrating 1 to 4
  Migrating 0 to 3

Signed-off-by: Larry Woodman<lwoodman@xxxxxxxxxx>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 47296fe..2bd13e9 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1012,6 +1012,9 @@ int do_migrate_pages(struct mm_struct *mm,
 		int dest = 0;
 
 		for_each_node_mask(s, tmp) {
+			/* no need to move if its already there */
+			if (node_isset(s, *to_nodes))
+				continue;
 			d = node_remap(s, *from_nodes, *to_nodes);
 			if (s == d)
 				continue;