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Date:   Tue, 21 May 2019 17:31:13 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Minchan Kim <minchan@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tim Murray <timmurray@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Daniel Colascione <dancol@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Sonny Rao <sonnyrao@google.com>,
        Brian Geffon <bgeffon@google.com>
Subject: Re: [RFC 5/7] mm: introduce external memory hinting API
Message-ID: <20190521153113.GA2235@redhat.com>
References: <20190520035254.57579-1-minchan@kernel.org>
 <20190520035254.57579-6-minchan@kernel.org>
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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.