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Date:   Fri, 6 Aug 2021 08:40:10 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, rientjes@google.com,
        willy@infradead.org, hannes@cmpxchg.org, guro@fb.com,
        riel@surriel.com, minchan@kernel.org, christian@brauner.io,
        hch@infradead.org, oleg@redhat.com, david@redhat.com,
        jannh@google.com, shakeelb@google.com, luto@kernel.org,
        christian.brauner@ubuntu.com, fweimer@redhat.com, jengelh@inai.de,
        timmurray@google.com, linux-api@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
Subject: Re: [PATCH v7 1/2] mm: introduce process_mrelease system call
Message-ID: <YQzZSmRqYmxFJ61y@dhcp22.suse.cz>
References: <20210805170859.2389276-1-surenb@google.com>
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On Thu 05-08-21 10:08:58, Suren Baghdasaryan wrote:
[...]
> +	/*
> +	 * If the task is dying and in the process of releasing its memory
> +	 * then get its mm.
> +	 */
> +	p = find_lock_task_mm(task);
> +	if (!p) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +	if (task != p) {
> +		get_task_struct(p);
> +		put_task_struct(task);
> +		task = p;
> +	}

Why do you need to take a reference to the p here? You are under
task_lock so this will not go away and you only need p to get your mm.

> +
> +	/* If the work has been done already, just exit with success */
> +	if (test_bit(MMF_OOM_SKIP, &task->mm->flags))
> +		goto put_task;

You want to release the task_lock

> +
> +	if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {

you want task_will_free_mem(p) and what is the point of the PF_KTHREAD
check?

> +		mm = task->mm;
> +		mmget(mm);

All you need is to make sure mm will not get released under your feet
once task_lock is released so mmgrab is the right thing to do here. The
address space can be torn down in parallel and that is OK and desirable.

I think you really want something like this:

	if (flags)
		return -EINVAL;
	
	pid = pidfd_get_pid(fd, &f_flags);
	if (IS_ERR(pid))
		return PTR_ERR(pid);
	task = get_pid_task(pid, PIDTYPE_PID);
	if (!task) {
		ret = -ESRCH;
		goto put_pid;
	}

	/*
	 * Make sure to chose a thread which still has a reference to mm
	 * during the group exit
	 */
	p = find_lock_task_mm(task);
	if (!p) {
		ret = -ESRCH;
		goto put_task;
	}

	mm = task->mm;
	mmgrab(mm);
	reap = true;
	/* If the work has been done already, just exit with success */
	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
		reap = false;
	} else if (!task_will_free_mem(p)) {
		reap = false;
		ret = -EINVAL;
	}
	task_unlock(p);

	if (!reap)
		goto dropmm;;

	/* Do the work*/


dropmm:
	mmdrop(mm);
put_task:
	put_task(task);
put_pid:
	put_pid(pid);

	return ret;

-- 
Michal Hocko
SUSE Labs