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 <hugh@veritas.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hirokazu Takahashi <taka@valinux.co.jp>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 6 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 13e9e7d8e49e..66d0e84cefa6 100644
--- 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 */