Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

[Joel Fernandes <joel@joelfernandes.org>]

On Sat, Mar 16, 2019 at 12:37:18PM -0700, Suren Baghdasaryan wrote:
> On Sat, Mar 16, 2019 at 11:57 AM Christian Brauner <christian@brauner.io> wrote:
> >
> > On Sat, Mar 16, 2019 at 11:00:10AM -0700, Daniel Colascione wrote:
> > > On Sat, Mar 16, 2019 at 10:31 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > > >
> > > > On Fri, Mar 15, 2019 at 11:49 AM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > >
> > > > > On Fri, Mar 15, 2019 at 07:24:28PM +0100, Christian Brauner wrote:
> > > > > [..]
> > > > > > > why do we want to add a new syscall (pidfd_wait) though? Why not just use
> > > > > > > standard poll/epoll interface on the proc fd like Daniel was suggesting.
> > > > > > > AFAIK, once the proc file is opened, the struct pid is essentially pinned
> > > > > > > even though the proc number may be reused. Then the caller can just poll.
> > > > > > > We can add a waitqueue to struct pid, and wake up any waiters on process
> > > > > > > death (A quick look shows task_struct can be mapped to its struct pid) and
> > > > > > > also possibly optimize it using Steve's TIF flag idea. No new syscall is
> > > > > > > needed then, let me know if I missed something?
> > > > > >
> > > > > > Huh, I thought that Daniel was against the poll/epoll solution?
> > > > >
> > > > > Hmm, going through earlier threads, I believe so now. Here was Daniel's
> > > > > reasoning about avoiding a notification about process death through proc
> > > > > directory fd: http://lkml.iu.edu/hypermail/linux/kernel/1811.0/00232.html
> > > > >
> > > > > May be a dedicated syscall for this would be cleaner after all.
> > > >
> > > > Ah, I wish I've seen that discussion before...
> > > > syscall makes sense and it can be non-blocking and we can use
> > > > select/poll/epoll if we use eventfd.
> > >
> > > Thanks for taking a look.
> > >
> > > > I would strongly advocate for
> > > > non-blocking version or at least to have a non-blocking option.
> > >
> > > Waiting for FD readiness is *already* blocking or non-blocking
> > > according to the caller's desire --- users can pass options they want
> > > to poll(2) or whatever. There's no need for any kind of special
> > > configuration knob or non-blocking option. We already *have* a
> > > non-blocking option that works universally for everything.
> > >
> > > As I mentioned in the linked thread, waiting for process exit should
> > > work just like waiting for bytes to appear on a pipe. Process exit
> > > status is just another blob of bytes that a process might receive. A
> > > process exit handle ought to be just another information source. The
> > > reason the unix process API is so awful is that for whatever reason
> > > the original designers treated processes as some kind of special kind
> > > of resource instead of fitting them into the otherwise general-purpose
> > > unix data-handling API. Let's not repeat that mistake.
> > >
> > > > Something like this:
> > > >
> > > > evfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
> > > > // register eventfd to receive death notification
> > > > pidfd_wait(pid_to_kill, evfd);
> > > > // kill the process
> > > > pidfd_send_signal(pid_to_kill, ...)
> > > > // tend to other things
> > >
> > > Now you've lost me. pidfd_wait should return a *new* FD, not wire up
> > > an eventfd.
> > >
> 
> Ok, I probably misunderstood your post linked by Joel. I though your
> original proposal was based on being able to poll a file under
> /proc/pid and then you changed your mind to have a separate syscall
> which I assumed would be a blocking one to wait for process exit.
> Maybe you can describe the new interface you are thinking about in
> terms of userspace usage like I did above? Several lines of code would
> explain more than paragraphs of text.

Hey, Thanks Suren for the eventfd idea. I agree with Daniel on this. The idea
from Daniel here is to wait for process death and exit events by just
referring to a stable fd, independent of whatever is going on in /proc.

What is needed is something like this (in highly pseudo-code form):

pidfd = opendir("/proc/<pid>",..);
wait_fd = pidfd_wait(pidfd);
read or poll wait_fd (non-blocking or blocking whichever)

wait_fd will block until the task has either died or reaped. In both these
cases, it can return a suitable string such as "dead" or "reaped" although an
integer with some predefined meaning is also Ok.

What that guarantees is, even if the task's PID has been reused, or the task
has already died or already died + reaped, all of these events cannot race
with the code above and the information passed to the user is race-free and
stable / guaranteed.

An eventfd seems to not fit well, because AFAICS passing the raw PID to
eventfd as in your example would still race since the PID could have been
reused by another process by the time the eventfd is created.

Also Andy's idea in [1] seems to use poll flags to communicate various tihngs
which is still not as explicit about the PID's status so that's a poor API
choice compared to the explicit syscall.

I am planning to work on a prototype patch based on Daniel's idea and post something
soon (chatted with Daniel about it and will reference him in the posting as
well), during this posting I will also summarize all the previous discussions
and come up with some tests as well.  I hope to have something soon.

Let me know if I hit all the points correctly and I hope we are all on the
same page.

Thanks!

 - Joel

[1] http://lkml.iu.edu/hypermail//linux/kernel/1212.0/00808.html