Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited process killing

[Daniel Colascione]

On Fri, Apr 12, 2019 at 7:14 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>
> On Thu, Apr 11, 2019 at 11:53 PM Michal Hocko <mhocko@xxxxxxxxxx> 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.

Just for clarity, checking the code, I think we already do this.
zap_other_threads sets SIGKILL pending on every thread in the group,
and we'll handle SIGKILL in the process of taking any page fault or
doing any system call, so I don't think it's actually possible for a
thread in a dying process to observe the SIGBUS that reaping in theory
can generate.