memcg: css_put after remove_list

mem_cgroup_uncharge_page does css_put on the mem_cgroup before uncharging from
it, and before removing page_cgroup from one of its lru lists: isn't there a
danger that struct mem_cgroup memory could be freed and reused before
completing that, so corrupting something?  Never seen it, and for all I know
there may be other constraints which make it impossible; but let's be
defensive and reverse the ordering there.

mem_cgroup_force_empty_list is safe because there's an extra css_get around
all its works; but even so, change its ordering the same way round, to help
get in the habit of doing it like this.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Balbir Singh <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: Hirokazu Takahashi <[email protected]>
Cc: YAMAMOTO Takashi <[email protected]>
Cc: Paul Menage <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 13e9e7d8e49e7f9004a66fd8d59a55b65e313f5c..66d0e84cefa6533174e08d7c86006953f4bbd68a 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -665,15 +665,15 @@ void mem_cgroup_uncharge_page(struct page *page)
                page_assign_page_cgroup(page, NULL);
                unlock_page_cgroup(page);
 
-               mem = pc->mem_cgroup;
-               css_put(&mem->css);
-               res_counter_uncharge(&mem->res, PAGE_SIZE);
-
                mz = page_cgroup_zoneinfo(pc);
                spin_lock_irqsave(&mz->lru_lock, flags);
                __mem_cgroup_remove_list(pc);
                spin_unlock_irqrestore(&mz->lru_lock, flags);
 
+               mem = pc->mem_cgroup;
+               res_counter_uncharge(&mem->res, PAGE_SIZE);
+               css_put(&mem->css);
+
                kfree(pc);
                return;
        }
@@ -774,9 +774,9 @@ retry:
                if (page_get_page_cgroup(page) == pc) {
                        page_assign_page_cgroup(page, NULL);
                        unlock_page_cgroup(page);
-                       css_put(&mem->css);
-                       res_counter_uncharge(&mem->res, PAGE_SIZE);
                        __mem_cgroup_remove_list(pc);
+                       res_counter_uncharge(&mem->res, PAGE_SIZE);
+                       css_put(&mem->css);
                        kfree(pc);
                } else {
                        /* racing uncharge: let page go then retry */