Re: WARNING in handle_mm_fault

[Johannes Weiner <hannes@cmpxchg.org>]

Hi Dmitry,

On Tue, Nov 24, 2015 at 02:50:26PM +0100, Dmitry Vyukov wrote:
> As a blind guess, I've added the following BUG into copy_process:
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b4dc490..c5667e8 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1620,6 +1620,8 @@ static struct task_struct *copy_process(unsigned
> long clone_flags,
>         trace_task_newtask(p, clone_flags);
>         uprobe_copy_process(p, clone_flags);
> 
> +       BUG_ON(p->memcg_may_oom);
> +
>         return p;

Thanks for your report.

I don't see how this could happen through the legitimate setters of
p->memcg_may_oom. Something must clobber it. What happens with the
following patch applied?

diff --git a/include/linux/sched.h b/include/linux/sched.h
index edad7a4..42e1285 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1463,9 +1463,11 @@ struct task_struct {
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
 	unsigned sched_migrated:1;
+	unsigned dummy_a:1;
 #ifdef CONFIG_MEMCG
 	unsigned memcg_may_oom:1;
 #endif
+	unsigned dummy_b:1;
 #ifdef CONFIG_MEMCG_KMEM
 	unsigned memcg_kmem_skip_account:1;
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index f97f2c4..ab6f7ba 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1617,6 +1617,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	trace_task_newtask(p, clone_flags);
 	uprobe_copy_process(p, clone_flags);
 
+	if (p->dummy_a || p->dummy_b || p->memcg_may_oom) {
+		printk(KERN_ALERT "dummy_a:%d dummy_b:%d memcg_may_oom:%d\n",
+		       p->dummy_a, p->dummy_b, p->memcg_may_oom);
+		BUG();
+	}
+
 	return p;
 
 bad_fork_cancel_cgroup:

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