[BUGFIX][PATCH] fix wrong VM_BUG_ON() in try_charge()'s mm->owner check

[Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>]

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I'm sorry for attached file, I have to use unusual mailer this time.
This is a fix for wrong VM_BUG_ON() for mm/memcontol.c

Thanks,
-Kame

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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 stroy 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: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

Index: linux-2.6.36/mm/memcontrol.c
===================================================================
--- linux-2.6.36.orig/mm/memcontrol.c
+++ linux-2.6.36/mm/memcontrol.c
@@ -1729,19 +1729,18 @@ 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;
 		}