Re: [RFC PATCH v2] Minimal non-child process exit notification support

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2018-11-01, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2018-10-29, Daniel Colascione <dancol@google.com> wrote:
> > This patch adds a new file under /proc/pid, /proc/pid/exithand.
> > Attempting to read from an exithand file will block until the
> > corresponding process exits, at which point the read will successfully
> > complete with EOF.  The file descriptor supports both blocking
> > operations and poll(2). It's intended to be a minimal interface for
> > allowing a program to wait for the exit of a process that is not one
> > of its children.
> > 
> > Why might we want this interface? Android's lmkd kills processes in
> > order to free memory in response to various memory pressure
> > signals. It's desirable to wait until a killed process actually exits
> > before moving on (if needed) to killing the next process. Since the
> > processes that lmkd kills are not lmkd's children, lmkd currently
> > lacks a way to wait for a process to actually die after being sent
> > SIGKILL; today, lmkd resorts to polling the proc filesystem pid
> > entry. This interface allow lmkd to give up polling and instead block
> > and wait for process death.
> 
> I agree with the need for this interface (with a few caveats), but there
> are a few points I'd like to make:
> 
>  * I don't think that making a new procfile is necessary. When you open
>    /proc/$pid you already have a handle for the underlying process, and
>    you can already poll to check whether the process has died (fstatat
>    fails for instance). What if we just used an inotify event to tell
>    userspace that the process has died -- to avoid userspace doing a
>    poll loop?
> 
>  * There is a fairly old interface called the proc_connector which gives
>    you global fork+exec+exit events (similar to kevents from FreeBSD
>    though much less full-featured). I was working on some patches to
>    extend proc_connector so that it could be used inside containers as
>    well as unprivileged users. This would be another way we could
>    implement this.
> 
> I'm really not a huge fan of the "blocking read" semantic (though if we
> have to have it, can we at least provide as much information as you get
> from proc_connector -- such as the exit status?). Also maybe we should
> integrate this into the exit machinery instead of this loop...

In addition, given that you've posted two patches in the similar vein
but as separate patchsets -- would you mind re-sending them as a single
patchset (with all the relevant folks added to Cc)?

If the idea is to extend /proc/$pid to allow for various
fd-as-process-handle operations (which I agree with in principle), then
they should be a single patchset. I'm also a bit cautious about how
many procfiles the eventual goal is to add.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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