1 files changed, 23 insertions, 11 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 15eb2273d80b..8c2835a9e192 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1902,10 +1902,6 @@ static void cgroup_migrate_add_src(struct css_set *src_cset,
 
         src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
-        /* nothing to do if this cset already belongs to the cgroup */
-        if (src_cgrp == dst_cgrp)
-                return;
-
         if (!list_empty(&src_cset->mg_preload_node))
                 return;
 
@@ -1920,13 +1916,14 @@ static void cgroup_migrate_add_src(struct css_set *src_cset,
 
 /**
  * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
- * @dst_cgrp: the destination cgroup
+ * @dst_cgrp: the destination cgroup (may be %NULL)
  * @preloaded_csets: list of preloaded source css_sets
  *
  * Tasks are about to be moved to @dst_cgrp and all the source css_sets
  * have been preloaded to @preloaded_csets.  This function looks up and
- * pins all destination css_sets, links each to its source, and put them on
- * @preloaded_csets.
+ * pins all destination css_sets, links each to its source, and append them
+ * to @preloaded_csets.  If @dst_cgrp is %NULL, the destination of each
+ * source css_set is assumed to be its cgroup on the default hierarchy.
  *
  * This function must be called after cgroup_migrate_add_src() has been
  * called on each migration source css_set.  After migration is performed
@@ -1937,19 +1934,34 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
                                       struct list_head *preloaded_csets)
 {
         LIST_HEAD(csets);
-        struct css_set *src_cset;
+        struct css_set *src_cset, *tmp_cset;
 
         lockdep_assert_held(&cgroup_mutex);
 
         /* look up the dst cset for each src cset and link it to src */
-        list_for_each_entry(src_cset, preloaded_csets, mg_preload_node) {
+        list_for_each_entry_safe(src_cset, tmp_cset, preloaded_csets, mg_preload_node) {
                 struct css_set *dst_cset;
 
-                dst_cset = find_css_set(src_cset, dst_cgrp);
+                dst_cset = find_css_set(src_cset,
+                                        dst_cgrp ?: src_cset->dfl_cgrp);
                 if (!dst_cset)
                         goto err;
 
                 WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);
+
+                /*
+                 * If src cset equals dst, it's noop.  Drop the src.
+                 * cgroup_migrate() will skip the cset too.  Note that we
+                 * can't handle src == dst as some nodes are used by both.
+                 */
+                if (src_cset == dst_cset) {
+                        src_cset->mg_src_cgrp = NULL;
+                        list_del_init(&src_cset->mg_preload_node);
+                        put_css_set(src_cset, false);
+                        put_css_set(dst_cset, false);
+                        continue;
+                }
+
                 src_cset->mg_dst_cset = dst_cset;
 
                 if (list_empty(&dst_cset->mg_preload_node))
@@ -1958,7 +1970,7 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
                         put_css_set(dst_cset, false);
         }
 
-        list_splice(&csets, preloaded_csets);
+        list_splice_tail(&csets, preloaded_csets);
         return 0;
 err:
         cgroup_migrate_finish(&csets);