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

[Oleg Nesterov]

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.

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

Oleg.