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
* Hiroyuki Kamezawa <[email protected]> [2010-11-03 00:10:50]:
> 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
>
Yes, that seems reasonable. If we race with try_to_unuse() and
the mm has no new owner we set mm->owner to NULL, in those cases it
makes no sense to charge.
Reviewed-by: Balbir Singh <[email protected]>
--
Three Cheers,
Balbir
On Wed, 3 Nov 2010, Hiroyuki Kamezawa wrote:
> 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, that's good: I've inlined it below with Balbir's Review,
my Ack, and a Cc: [email protected].
Hugh
[PATCH] memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check
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 <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Reported-by: Hugh Dickins <[email protected]>
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Reviewed-by: Balbir Singh <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: [email protected]
---
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;
}
On Thu, Nov 04, 2010 at 10:31:58AM -0700, Hugh Dickins wrote:
> On Wed, 3 Nov 2010, Hiroyuki Kamezawa wrote:
> > 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, that's good: I've inlined it below with Balbir's Review,
> my Ack, and a Cc: [email protected].
>
> Hugh
>
>
> [PATCH] memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check
>
> 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 <[email protected]>
> Cc: KOSAKI Motohiro <[email protected]>
> Reported-by: Hugh Dickins <[email protected]>
> Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
> Reviewed-by: Balbir Singh <[email protected]>
> Acked-by: Hugh Dickins <[email protected]>
> Cc: [email protected]
Reviewed-by: Johannes Weiner <[email protected]>