From: KAMEZAWA Hiroyuki Date: Wed, 29 Dec 2010 22:07:11 +0000 (-0800) Subject: memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check X-Git-Tag: firefly_0821_release~9833^2~71^2^2~19 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=e606509bc1d7f19fccf078907d5421b3a95bd659;p=firefly-linux-kernel-4.4.55.git memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check commit ebb76ce16daf6908dc030dec1c00827d37129fe5 upstream. 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 Cc: KOSAKI Motohiro Reported-by: Hugh Dickins Reported-by: Thomas Meyer Signed-off-by: KAMEZAWA Hiroyuki Reviewed-by: Balbir Singh Signed-off-by: Hugh Dickins Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e6aadd65a71a..0f900de4c676 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1730,19 +1730,18 @@ 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; }