memcg: close race between charge and putback

There is a potential race between a thread charging a page and another
thread putting it back to the LRU list:

charge:                         putback:
SetPageCgroupUsed               SetPageLRU
PageLRU && add to memcg LRU     PageCgroupUsed && add to memcg LRU

The order of setting one flag and checking the other is crucial, otherwise
the charge may observe !PageLRU while the putback observes !PageCgroupUsed
and the page is not linked to the memcg LRU at all.

Global memory pressure may fix this by trying to isolate and putback the
page for reclaim, where that putback would link it to the memcg LRU again.
 Without that, the memory cgroup is undeletable due to a charge whose
physical page can not be found and moved out.

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
Cc: Ying Han <yinghan@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 60c2bd04ee63c054c6009cbc65a6d15b3ab4fb00..44f4311c07221f36b2a009d7b01acc19c4128c2f 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -994,6 +994,16 @@ void mem_cgroup_add_lru_list(struct page *page, enum lru_list lru)
                return;
        pc = lookup_page_cgroup(page);
        VM_BUG_ON(PageCgroupAcctLRU(pc));
+       /*
+        * putback:                             charge:
+        * SetPageLRU                           SetPageCgroupUsed
+        * smp_mb                               smp_mb
+        * PageCgroupUsed && add to memcg LRU   PageLRU && add to memcg LRU
+        *
+        * Ensure that one of the two sides adds the page to the memcg
+        * LRU during a race.
+        */
+       smp_mb();
        if (!PageCgroupUsed(pc))
                return;
        /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
@@ -1045,7 +1055,16 @@ static void mem_cgroup_lru_add_after_commit(struct page *page)
        unsigned long flags;
        struct zone *zone = page_zone(page);
        struct page_cgroup *pc = lookup_page_cgroup(page);
-
+       /*
+        * putback:                             charge:
+        * SetPageLRU                           SetPageCgroupUsed
+        * smp_mb                               smp_mb
+        * PageCgroupUsed && add to memcg LRU   PageLRU && add to memcg LRU
+        *
+        * Ensure that one of the two sides adds the page to the memcg
+        * LRU during a race.
+        */
+       smp_mb();
        /* taking care of that the page is added to LRU while we commit it */
        if (likely(!PageLRU(page)))
                return;