mm: simplify lock_page_memcg()

Now that migration doesn't clear page->mem_cgroup of live pages anymore,
it's safe to make lock_page_memcg() and the memcg stat functions take
pages, and spare the callers from memcg objects.

[akpm@linux-foundation.org: fix warnings]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 14 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 64506b2eef34..3e4199830456 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1690,7 +1690,7 @@ cleanup:
  * This function protects unlocked LRU pages from being moved to
  * another cgroup and stabilizes their page->mem_cgroup binding.
  */
-struct mem_cgroup *lock_page_memcg(struct page *page)
+void lock_page_memcg(struct page *page)
 {
 	struct mem_cgroup *memcg;
 	unsigned long flags;
@@ -1699,25 +1699,18 @@ struct mem_cgroup *lock_page_memcg(struct page *page)
 	 * The RCU lock is held throughout the transaction.  The fast
 	 * path can get away without acquiring the memcg->move_lock
 	 * because page moving starts with an RCU grace period.
-	 *
-	 * The RCU lock also protects the memcg from being freed when
-	 * the page state that is going to change is the only thing
-	 * preventing the page from being uncharged.
-	 * E.g. end-writeback clearing PageWriteback(), which allows
-	 * migration to go ahead and uncharge the page before the
-	 * account transaction might be complete.
 	 */
 	rcu_read_lock();
 
 	if (mem_cgroup_disabled())
-		return NULL;
+		return;
 again:
 	memcg = page->mem_cgroup;
 	if (unlikely(!memcg))
-		return NULL;
+		return;
 
 	if (atomic_read(&memcg->moving_account) <= 0)
-		return memcg;
+		return;
 
 	spin_lock_irqsave(&memcg->move_lock, flags);
 	if (memcg != page->mem_cgroup) {
@@ -1733,16 +1726,18 @@ again:
 	memcg->move_lock_task = current;
 	memcg->move_lock_flags = flags;
 
-	return memcg;
+	return;
 }
 EXPORT_SYMBOL(lock_page_memcg);
 
 /**
  * unlock_page_memcg - unlock a page->mem_cgroup binding
- * @memcg: the memcg returned by lock_page_memcg()
+ * @page: the page
  */
-void unlock_page_memcg(struct mem_cgroup *memcg)
+void unlock_page_memcg(struct page *page)
 {
+	struct mem_cgroup *memcg = page->mem_cgroup;
+
 	if (memcg && memcg->move_lock_task == current) {
 		unsigned long flags = memcg->move_lock_flags;