mm: memcontrol: update mem_cgroup_page_lruvec() documentation

[deliverable/linux.git] / mm / memcontrol.c

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 975207a9cc65581fb0bcae9782db7229b0941f5a..b495f29d4746a47deb5b5575c93a6d3e1b041710 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1262,9 +1262,13 @@ out:
 }
 
 /**
- * mem_cgroup_page_lruvec - return lruvec for adding an lru page
+ * mem_cgroup_page_lruvec - return lruvec for isolating/putting an LRU page
  * @page: the page
  * @zone: zone of the page
+ *
+ * This function is only safe when following the LRU page isolation
+ * and putback protocol: the LRU lock must be held, and the page must
+ * either be PageLRU() or the caller must have isolated/allocated it.
  */
 struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone)
 {
@@ -1282,13 +1286,9 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct zone *zone)
        memcg = pc->mem_cgroup;
 
        /*
-        * Surreptitiously switch any uncharged offlist page to root:
-        * an uncharged page off lru does nothing to secure
-        * its former mem_cgroup from sudden removal.
-        *
-        * Our caller holds lru_lock, and PageCgroupUsed is updated
-        * under page_cgroup lock: between them, they make all uses
-        * of pc->mem_cgroup safe.
+        * Swapcache readahead pages are added to the LRU - and
+        * possibly migrated - before they are charged.  Ensure
+        * pc->mem_cgroup is sane.
         */
        if (!PageLRU(page) && !PageCgroupUsed(pc) && memcg != root_mem_cgroup)
                pc->mem_cgroup = memcg = root_mem_cgroup;