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

[Daniel Colascione]

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.