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From:   Daniel Colascione <dancol@google.com>
Date:   Fri, 12 Apr 2019 07:14:30 -0700
Message-ID: <CAKOZuetQH1rVtPdMNgw0sdnzWidd6v9eCWscRiOb7Y+3-JQ14Q@mail.gmail.com>
Subject: Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited
 process killing
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        yuzhoujian@didichuxing.com,
        Souptick Joarder <jrdr.linux@gmail.com>,
        Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Shakeel Butt <shakeelb@google.com>,
        Christian Brauner <christian@brauner.io>,
        Minchan Kim <minchan@kernel.org>,
        Tim Murray <timmurray@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Jann Horn <jannh@google.com>, linux-mm <linux-mm@kvack.org>,
        lsf-pc@lists.linux-foundation.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
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On Thu, Apr 11, 2019 at 11:53 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 11-04-19 08:33:13, Matthew Wilcox wrote:
> > On Wed, Apr 10, 2019 at 06:43:53PM -0700, Suren Baghdasaryan wrote:
> > > Add new SS_EXPEDITE flag to be used when sending SIGKILL via
> > > pidfd_send_signal() syscall to allow expedited memory reclaim of the
> > > victim process. The usage of this flag is currently limited to SIGKILL
> > > signal and only to privileged users.
> >
> > What is the downside of doing expedited memory reclaim?  ie why not do it
> > every time a process is going to die?
>
> Well, you are tearing down an address space which might be still in use
> because the task not fully dead yeat. So there are two downsides AFAICS.
> Core dumping which will not see the reaped memory so the resulting

Test for SIGNAL_GROUP_COREDUMP before doing any of this then. If you
try to start a core dump after reaping begins, too bad: you could have
raced with process death anyway.

> coredump might be incomplete. And unexpected #PF/gup on the reaped
> memory will result in SIGBUS.

It's a dying process. Why even bother returning from the fault
handler? Just treat that situation as a thread exit. There's no need
to make this observable to userspace at all.