From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
At __mem_cgroup_try_charge(), VM_BUG_ON(!mm->owner) is checked.
But as commented in mem_cgroup_from_task(), mm->owner can be NULL
in some racy case. This check of VM_BUG_ON() is bad.
A possible story to hit this is at swapoff()->try_to_unuse(). It passes
mm_struct to mem_cgroup_try_charge_swapin() while mm->owner is NULL. If we
can't get proper mem_cgroup from swap_cgroup information, mm->owner is used
as charge target and we see NULL.
Cc: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reported-by: Thomas Meyer <thomas@xxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Reviewed-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
Sorry, I hit this on 2.6.36, and we lined up this patch early in
November, but never really pushed it: now Thomas hit it on 37-rc7.
mm/memcontrol.c | 19 +++++++++----------
1 file changed, 9 insertions(+), 10 deletions(-)
--- 2.6.37-rc8/mm/memcontrol.c 2010-11-29 22:29:32.000000000 -0800
+++ linux/mm/memcontrol.c 2010-12-28 21:42:29.000000000 -0800
@@ -1925,19 +1925,18 @@ again:
rcu_read_lock();
p = rcu_dereference(mm->owner);
- VM_BUG_ON(!p);
/*
- * because we don't have task_lock(), "p" can exit while
- * we're here. In that case, "mem" can point to root
- * cgroup but never be NULL. (and task_struct itself is freed
- * by RCU, cgroup itself is RCU safe.) Then, we have small
- * risk here to get wrong cgroup. But such kind of mis-account
- * by race always happens because we don't have cgroup_mutex().
- * It's overkill and we allow that small race, here.
+ * Because we don't have task_lock(), "p" can exit.
+ * In that case, "mem" can point to root or p can be NULL with
+ * race with swapoff. Then, we have small risk of mis-accouning.
+ * But such kind of mis-account by race always happens because
+ * we don't have cgroup_mutex(). It's overkill and we allo that
+ * small race, here.
+ * (*) swapoff at el will charge against mm-struct not against
+ * task-struct. So, mm->owner can be NULL.
*/
mem = mem_cgroup_from_task(p);
- VM_BUG_ON(!mem);
- if (mem_cgroup_is_root(mem)) {
+ if (!mem || mem_cgroup_is_root(mem)) {
rcu_read_unlock();
goto done;
}
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