linux-kernel - Re: [PATCH v6 1/2] mm: introduce process

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Message-ID: <YQuO36AeQUwsAyU1@dhcp22.suse.cz>
Date:   Thu, 5 Aug 2021 09:10:23 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, rientjes@...gle.com,
        willy@...radead.org, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, christian@...uner.io,
        hch@...radead.org, oleg@...hat.com, david@...hat.com,
        jannh@...gle.com, shakeelb@...gle.com, luto@...nel.org,
        christian.brauner@...ntu.com, fweimer@...hat.com, jengelh@...i.de,
        timmurray@...gle.com, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v6 1/2] mm: introduce process_mrelease system call

On Wed 04-08-21 11:50:03, Suren Baghdasaryan wrote:
[...]
> +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> +{
> +#ifdef CONFIG_MMU
> +	struct mm_struct *mm = NULL;
> +	struct task_struct *task;
> +	unsigned int f_flags;
> +	struct pid *pid;
> +	long ret = 0;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	pid = pidfd_get_pid(pidfd, &f_flags);
> +	if (IS_ERR(pid))
> +		return PTR_ERR(pid);
> +
> +	task = get_pid_task(pid, PIDTYPE_PID);
> +	if (!task) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +
> +	/*
> +	 * If the task is dying and in the process of releasing its memory
> +	 * then get its mm.
> +	 */
> +	task = find_lock_task_mm(task);

You want a different task_struct because the returned one might be
different from the given one and you already hold a reference which you
do not want to leak

> +	if (!task) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +	if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
> +		mm = task->mm;
> +		mmget(mm);
> +	}
> +	task_unlock(task);
> +	if (!mm) {
> +		ret = -EINVAL;
> +		goto put_task;
> +	}
> +
> +	if (test_bit(MMF_OOM_SKIP, &mm->flags))
> +		goto put_mm;

This is too late to check for MMF_OOM_SKIP. task_will_free_mem will fail
with the flag being set. I believe you want something like the
following:

	p = find_lock_task_mm(task);
	mm = p->mm;

	/* The work has been done already */
	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
		task_unlock(p);	
		goto put_task;
	}

	i
	if (!task_will_free_mem(p)) {
		task_unlock(p);	
		goto put_task;
	}

	mmget(mm);
	task_unlock(p);


> +
> +	if (mmap_read_lock_killable(mm)) {
> +		ret = -EINTR;
> +		goto put_mm;
> +	}
> +	if (!__oom_reap_task_mm(mm))
> +		ret = -EAGAIN;
> +	mmap_read_unlock(mm);
> +
> +put_mm:
> +	mmput(mm);
> +put_task:
> +	put_task_struct(task);
> +put_pid:
> +	put_pid(pid);
> +	return ret;
> +#else
> +	return -ENOSYS;
> +#endif /* CONFIG_MMU */
> +}
> -- 
> 2.32.0.554.ge1b32706d8-goog

-- 
Michal Hocko
SUSE Labs