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memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check

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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 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@mxp.nes.nec.co.jp>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reported-by: Hugh Dickins <hughd@google.com>
Reported-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
KAMEZAWA Hiroyuki 2010-12-29 14:07:11 -08:00 committed by Linus Torvalds
parent b83be6f20a
commit ebb76ce16d
1 changed files with 9 additions and 10 deletions
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19
mm/memcontrol.c
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@ -1925,19 +1925,18 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
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;
}