Re: [PATCH 4/6] mm, oom: skip vforked tasks from being selected

[Oleg Nesterov]

On 05/30, Michal Hocko wrote:
>
> Make sure to not select vforked task as an oom victim by checking
> vfork_done in oom_badness.

I agree, this look like a good change to me... But.

> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -176,11 +176,13 @@ unsigned long oom_badness(struct task_struct *p, struct 
> mem_cgroup *memcg,
>  
>       /*
>        * Do not even consider tasks which are explicitly marked oom
> -      * unkillable or have been already oom reaped.
> +      * unkillable or have been already oom reaped or the are in
> +      * the middle of vfork
>        */
>       adj = (long)p->signal->oom_score_adj;
>       if (adj == OOM_SCORE_ADJ_MIN ||
> -                     test_bit(MMF_OOM_REAPED, &p->mm->flags)) {
> +                     test_bit(MMF_OOM_REAPED, &p->mm->flags) ||
> +                     p->vfork_done) {

I don't think we can trust vfork_done != NULL.

copy_process() doesn't disallow CLONE_VFORK without CLONE_VM, so with this patch
it would be trivial to make the exploit which hides a memory hog from 
oom-killer.

So perhaps we need something like

                bool in_vfork(p)
                {
                        return  p->vfork_done &&
                                p->real_parent->mm == mm;

                        
                }

task_lock() is not enough if CLONE_VM was used along with CLONE_PARENT... so 
this
also needs rcu_read_lock() to access ->real_parent.

Or I am totally confused?

Oleg.