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

[Christian Brauner <christian@brauner.io>]

On November 1, 2018 8:06:52 AM GMT+01:00, Aleksa Sarai <cyphar@cyphar.com> wrote:
>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)?

Please make sure to run get_maintainers.pl against your patches
if you haven't already done so to make sure that the right people are 
Cc'ed.
I would suggest to a least Cc Eric, Serge, Andy, Kees, and Oleg.

>
>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.