cgroup: track migration context in cgroup_mgctx

cgroup migration is performed in four steps - css_set preloading,
addition of target tasks, actual migration, and clean up.  A list
named preloaded_csets is used to track the preloading.  This is a bit
too restricted and the code is already depending on the subtlety that
all source css_sets appear before destination ones.

Let's create struct cgroup_mgctx which keeps track of everything
during migration.  Currently, it has separate preload lists for source
and destination csets and also embeds cgroup_taskset which is used
during the actual migration.  This moves struct cgroup_taskset
definition to cgroup-internal.h.

This patch doesn't cause any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

kernel/cgroup/cgroup-internal.h

@@ -26,6 +26,61 @@ struct cgrp_cset_link {
 	struct list_head	cgrp_link;
 };
 
+/* used to track tasks and csets during migration */
+struct cgroup_taskset {
+	/* the src and dst cset list running through cset->mg_node */
+	struct list_head	src_csets;
+	struct list_head	dst_csets;
+
+	/* the subsys currently being processed */
+	int			ssid;
+
+	/*
+	 * Fields for cgroup_taskset_*() iteration.
+	 *
+	 * Before migration is committed, the target migration tasks are on
+	 * ->mg_tasks of the csets on ->src_csets.  After, on ->mg_tasks of
+	 * the csets on ->dst_csets.  ->csets point to either ->src_csets
+	 * or ->dst_csets depending on whether migration is committed.
+	 *
+	 * ->cur_csets and ->cur_task point to the current task position
+	 * during iteration.
+	 */
+	struct list_head	*csets;
+	struct css_set		*cur_cset;
+	struct task_struct	*cur_task;
+};
+
+/* migration context also tracks preloading */
+struct cgroup_mgctx {
+	/*
+	 * Preloaded source and destination csets.  Used to guarantee
+	 * atomic success or failure on actual migration.
+	 */
+	struct list_head	preloaded_src_csets;
+	struct list_head	preloaded_dst_csets;
+
+	/* tasks and csets to migrate */
+	struct cgroup_taskset	tset;
+};
+
+#define CGROUP_TASKSET_INIT(tset)						\
+{										\
+	.src_csets		= LIST_HEAD_INIT(tset.src_csets),		\
+	.dst_csets		= LIST_HEAD_INIT(tset.dst_csets),		\
+	.csets			= &tset.src_csets,				\
+}
+
+#define CGROUP_MGCTX_INIT(name)							\
+{										\
+	LIST_HEAD_INIT(name.preloaded_src_csets),				\
+	LIST_HEAD_INIT(name.preloaded_dst_csets),				\
+	CGROUP_TASKSET_INIT(name.tset),						\
+}
+
+#define DEFINE_CGROUP_MGCTX(name)						\
+	struct cgroup_mgctx name = CGROUP_MGCTX_INIT(name)
+
 struct cgroup_sb_opts {
 	u16 subsys_mask;
 	unsigned int flags;
@@ -112,13 +167,12 @@ struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
 			       struct cgroup_namespace *ns);
 
 bool cgroup_may_migrate_to(struct cgroup *dst_cgrp);
-void cgroup_migrate_finish(struct list_head *preloaded_csets);
-void cgroup_migrate_add_src(struct css_set *src_cset,
-			    struct cgroup *dst_cgrp,
-			    struct list_head *preloaded_csets);
-int cgroup_migrate_prepare_dst(struct list_head *preloaded_csets);
+void cgroup_migrate_finish(struct cgroup_mgctx *mgctx);
+void cgroup_migrate_add_src(struct css_set *src_cset, struct cgroup *dst_cgrp,
+			    struct cgroup_mgctx *mgctx);
+int cgroup_migrate_prepare_dst(struct cgroup_mgctx *mgctx);
 int cgroup_migrate(struct task_struct *leader, bool threadgroup,
-		   struct cgroup_root *root);
+		   struct cgroup_mgctx *mgctx, struct cgroup_root *root);
 
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 		       bool threadgroup);

kernel/cgroup/cgroup-v1.c

@@ -87,7 +87,7 @@ EXPORT_SYMBOL_GPL(cgroup_attach_task_all);
  */
 int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 {
-	LIST_HEAD(preloaded_csets);
+	DEFINE_CGROUP_MGCTX(mgctx);
 	struct cgrp_cset_link *link;
 	struct css_task_iter it;
 	struct task_struct *task;
@@ -106,10 +106,10 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 	/* all tasks in @from are being moved, all csets are source */
 	spin_lock_irq(&css_set_lock);
 	list_for_each_entry(link, &from->cset_links, cset_link)
-		cgroup_migrate_add_src(link->cset, to, &preloaded_csets);
+		cgroup_migrate_add_src(link->cset, to, &mgctx);
 	spin_unlock_irq(&css_set_lock);
 
-	ret = cgroup_migrate_prepare_dst(&preloaded_csets);
+	ret = cgroup_migrate_prepare_dst(&mgctx);
 	if (ret)
 		goto out_err;
 
@@ -125,14 +125,14 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 		css_task_iter_end(&it);
 
 		if (task) {
-			ret = cgroup_migrate(task, false, to->root);
+			ret = cgroup_migrate(task, false, &mgctx, to->root);
 			if (!ret)
 				trace_cgroup_transfer_tasks(to, task, false);
 			put_task_struct(task);
 		}
 	} while (task && !ret);
 out_err:
-	cgroup_migrate_finish(&preloaded_csets);
+	cgroup_migrate_finish(&mgctx);
 	percpu_up_write(&cgroup_threadgroup_rwsem);
 	mutex_unlock(&cgroup_mutex);
 	return ret;