On Thu, Aug 20, 2020 at 01:13:49PM +0200, Michal Hocko wrote: > On Thu 20-08-20 12:55:56, Oleg Nesterov wrote: > > On 08/19, Suren Baghdasaryan wrote: > > > > > > Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely > > > used the additional mutex lock in that path of the clone() syscall should > > > not affect its overall performance. Clearing the MMF_PROC_SHARED flag > > > (when the last process sharing the mm exits) is left out of this patch to > > > keep it simple and because it is believed that this threading model is > > > rare. > > > > vfork() ? > > Could you be more specific?
vfork() implies CLONE_VM but !CLONE_THREAD. The way this patch is written the mutex lock will be taken every time you do a vfork(). (It's honestly also debatable whether it's that rare. For one, userspace stuff I maintain uses it too (see [1]). [1]: https://github.com/lxc/lxc/blob/9d3b7c97f0443adc9f0b0438437657ab42f5a1c3/src/lxc/start.c#L1676 ) > > > > --- a/kernel/fork.c > > > +++ b/kernel/fork.c > > > @@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, > > > struct task_struct *tsk) > > > if (clone_flags & CLONE_VM) { > > > mmget(oldmm); > > > mm = oldmm; > > > + if (!(clone_flags & CLONE_SIGHAND)) { > > > > I agree with Christian, you need CLONE_THREAD > > This was my suggestion to Suren, likely because I've misrememberd which > clone flag is responsible for the signal delivery. But now, after double > checking we do explicitly disallow CLONE_SIGHAND && !CLONE_VM. So > CLONE_THREAD is the right thing to check. > > > > + /* We need to synchronize with __set_oom_adj */ > > > + mutex_lock(&oom_adj_lock); > > > + set_bit(MMF_PROC_SHARED, &mm->flags); > > > + /* Update the values in case they were changed after > > > copy_signal */ > > > + tsk->signal->oom_score_adj = > > > current->signal->oom_score_adj; > > > + tsk->signal->oom_score_adj_min = > > > current->signal->oom_score_adj_min; > > > + mutex_unlock(&oom_adj_lock); > > > > I don't understand how this can close the race with __set_oom_adj... > > > > What if __set_oom_adj() is called right after mutex_unlock() ? It will see > > MMF_PROC_SHARED, but for_each_process() won't find the new child until > > copy_process() does list_add_tail_rcu(&p->tasks, &init_task.tasks) ? > > Good point. Then we will have to move this thing there. I was toying with moving this into sm like: static inline copy_oom_score(unsigned long flags, struct task_struct *tsk) trying to rely on set_bit() and test_bit() in copy_mm() being atomic and then calling it where Oleg said after the point of no return. Christian
