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

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Message-ID: <20190527233306.GE6879@google.com>
Date:   Tue, 28 May 2019 08:33:06 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>
Subject: Re: [RFC 5/7] mm: introduce external memory hinting API

On Mon, May 27, 2019 at 05:12:02PM +0200, Oleg Nesterov wrote:
> On 05/27, Minchan Kim wrote:
> >
> > > 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?
> 
> The leader can exit (call sys_exit(), not sys_exit_group()), this won't affect
> other threads. In this case the process is still alive even if the leader thread
> is zombie. That is why we have find_lock_task_mm().

Thanks for clarification, Oleg. Then, Let me have a further question.

It means process_vm_readv, move_pages have same problem too because find_task_by_vpid
can return a zomebie leader and next line checks for mm_struct validation makes a
failure. My understand is correct? If so, we need to fix all places.