Re: [RFC 5/7] mm: introduce external memory hinting API

[Minchan Kim]

Sorry for the late response. I miseed your comment. :(

On Tue, May 21, 2019 at 05:31:13PM +0200, Oleg Nesterov wrote:
> On 05/20, Minchan Kim wrote:
> >
> > +	rcu_read_lock();
> > +	tsk = pid_task(pid, PIDTYPE_PID);
> > +	if (!tsk) {
> > +		rcu_read_unlock();
> > +		goto err;
> > +	}
> > +	get_task_struct(tsk);
> > +	rcu_read_unlock();
> > +	mm = mm_access(tsk, PTRACE_MODE_ATTACH_REALCREDS);
> > +	if (!mm || IS_ERR(mm)) {
> > +		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> > +		if (ret == -EACCES)
> > +			ret = -EPERM;
> > +		goto err;
> > +	}
> > +	ret = madvise_core(tsk, start, len_in, behavior);
> 
> IIUC, madvise_core(tsk) plays with tsk->mm->mmap_sem. But this tsk can exit and
> nullify its ->mm right after mm_access() succeeds.

You're absolutely right. I will fix it via passing mm_struct instead of
task_struct.

Thanks!

> 
> another problem is that pid_task(pid) can return a zombie leader, in this case
> mm_access() will fail while it shouldn't.

I'm sorry. I didn't notice that. However, I couldn't understand your point. 
Why do you think mm_access shouldn't fail even though pid_task returns
a zombie leader? I thought it's okay since the target process is exiting
so hinting operation would be meaniness for the process.