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

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Message-ID: <20190528072344.GO1658@dhcp22.suse.cz>
Date:   Tue, 28 May 2019 09:23:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        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 Tue 28-05-19 08:33:06, Minchan Kim wrote:
> 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.

Isn't that a problem of most callers of get_task_mm? Shouldn't we fix it
turning it into find_lock_task_mm?
-- 
Michal Hocko
SUSE Labs