1 files changed, 31 insertions, 12 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3df3c04d73ab..e09741af816f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1611,9 +1611,13 @@ cleanup:
  * @page: the page
  *
  * This function protects unlocked LRU pages from being moved to
- * another cgroup and stabilizes their page->mem_cgroup binding.
+ * another cgroup.
+ *
+ * It ensures lifetime of the returned memcg. Caller is responsible
+ * for the lifetime of the page; __unlock_page_memcg() is available
+ * when @page might get freed inside the locked section.
  */
-void lock_page_memcg(struct page *page)
+struct mem_cgroup *lock_page_memcg(struct page *page)
 {
         struct mem_cgroup *memcg;
         unsigned long flags;
@@ -1622,18 +1626,24 @@ void 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 itself from being freed. E.g. writeback
+         * doesn't hold a page reference and relies on PG_writeback to
+         * keep off truncation, migration and so forth.
+         */
         rcu_read_lock();
 
         if (mem_cgroup_disabled())
-                return;
+                return NULL;
 again:
         memcg = page->mem_cgroup;
         if (unlikely(!memcg))
-                return;
+                return NULL;
 
         if (atomic_read(&memcg->moving_account) <= 0)
-                return;
+                return memcg;
 
         spin_lock_irqsave(&memcg->move_lock, flags);
         if (memcg != page->mem_cgroup) {
@@ -1649,18 +1659,18 @@ again:
         memcg->move_lock_task = current;
         memcg->move_lock_flags = flags;
 
-        return;
+        return memcg;
 }
 EXPORT_SYMBOL(lock_page_memcg);
 
 /**
- * unlock_page_memcg - unlock a page->mem_cgroup binding
- * @page: the page
+ * __unlock_page_memcg - unlock and unpin a memcg
+ * @memcg: the memcg
+ *
+ * Unlock and unpin a memcg returned by lock_page_memcg().
  */
-void unlock_page_memcg(struct page *page)
+void __unlock_page_memcg(struct mem_cgroup *memcg)
 {
-        struct mem_cgroup *memcg = page->mem_cgroup;
-
         if (memcg && memcg->move_lock_task == current) {
                 unsigned long flags = memcg->move_lock_flags;
 
@@ -1672,6 +1682,15 @@ void unlock_page_memcg(struct page *page)
 
         rcu_read_unlock();
 }
+
+/**
+ * unlock_page_memcg - unlock a page->mem_cgroup binding
+ * @page: the page
+ */
+void unlock_page_memcg(struct page *page)
+{
+        __unlock_page_memcg(page->mem_cgroup);
+}
 EXPORT_SYMBOL(unlock_page_memcg);
 
 /*