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

[Daniel Colascione <dancol@google.com>]

On Sun, Mar 17, 2019 at 4:42 AM Christian Brauner <christian@brauner.io> wrote:
>
> On Sat, Mar 16, 2019 at 09:53:06PM -0400, Joel Fernandes wrote:
> > 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.

I want to return a siginfo_t: we already use this structure in other
contexts to report exit status.

> > 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.

Thanks.

> Having pidfd_wait() return another fd will make the syscall harder to
> swallow for a lot of people I reckon.
> What exactly prevents us from making the pidfd itself readable/pollable
> for the exit staus? They are "special" fds anyway. I would really like
> to avoid polluting the api with multiple different types of fds if possible.

If pidfds had been their own file type, I'd agree with you. But pidfds
are directories, which means that we're beholden to make them behave
like directories normally do. I'd rather introduce another FD than
heavily overload the semantics of a directory FD in one particular
context. In no other circumstances are directory FDs also weird
IO-data sources. Our providing a facility to get a new FD to which we
*can* give pipe-like behavior does no harm and *usage* cleaner and
easier to reason about.