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Date:   Fri, 12 Apr 2019 07:14:30 -0700
From:   Daniel Colascione <>
To:     Michal Hocko <>
Cc:     Matthew Wilcox <>,
        Suren Baghdasaryan <>,
        Andrew Morton <>,
        David Rientjes <>,,
        Souptick Joarder <>,
        Roman Gushchin <>,
        Johannes Weiner <>,
        Tetsuo Handa <>,
        "Eric W. Biederman" <>,
        Shakeel Butt <>,
        Christian Brauner <>,
        Minchan Kim <>,
        Tim Murray <>,
        Joel Fernandes <>,
        Jann Horn <>, linux-mm <>,,
        linux-kernel <>,
        Android Kernel Team <>
Subject: Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited
 process killing

On Thu, Apr 11, 2019 at 11:53 PM Michal Hocko <> 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.

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