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

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Mar 20, 2019 at 12:10:23AM +0100, Christian Brauner wrote:
> On Tue, Mar 19, 2019 at 03:48:32PM -0700, Daniel Colascione wrote:
> > On Tue, Mar 19, 2019 at 3:14 PM Christian Brauner <christian@brauner.io> wrote:
> > > So I dislike the idea of allocating new inodes from the procfs super
> > > block. I would like to avoid pinning the whole pidfd concept exclusively
> > > to proc. The idea is that the pidfd API will be useable through procfs
> > > via open("/proc/<pid>") because that is what users expect and really
> > > wanted to have for a long time. So it makes sense to have this working.
> > > But it should really be useable without it. That's why translate_pid()
> > > and pidfd_clone() are on the table.  What I'm saying is, once the pidfd
> > > api is "complete" you should be able to set CONFIG_PROCFS=N - even
> > > though that's crazy - and still be able to use pidfds. This is also a
> > > point akpm asked about when I did the pidfd_send_signal work.
> > 
> > I agree that you shouldn't need CONFIG_PROCFS=Y to use pidfds. One
> > crazy idea that I was discussing with Joel the other day is to just
> > make CONFIG_PROCFS=Y mandatory and provide a new get_procfs_root()
> > system call that returned, out of thin air and independent of the
> > mount table, a procfs root directory file descriptor for the caller's
> > PID namspace and suitable for use with openat(2).
> 
> Even if this works I'm pretty sure that Al and a lot of others will not
> be happy about this. A syscall to get an fd to /proc? That's not going
> to happen and I don't see the need for a separate syscall just for that.
> (I do see the point of making CONFIG_PROCFS=y the default btw.)

I think his point here was that he wanted a handle to procfs no matter where
it was mounted and then can later use openat on that. Agreed that it may be
unnecessary unless there is a usecase for it, and especially if the /proc
directory being the defacto mountpoint for procfs is a universal convention.

> Inode allocation from the procfs mount for the file descriptors Joel
> wants is not correct. Their not really procfs file descriptors so this
> is a nack. We can't just hook into proc that way.

I was not particular about using procfs mount for the FDs but that's the only
way I knew how to do it until you pointed out anon_inode (my grep skills
missed that), so thank you!

> > C'mon: /proc is used by everyone today and almost every program breaks
> > if it's not around. The string "/proc" is already de facto kernel ABI.
> > Let's just drop the pretense of /proc being optional and bake it into
> > the kernel proper, then give programs a way to get to /proc that isn't
> > tied to any particular mount configuration. This way, we don't need a
> > translate_pid(), since callers can just use procfs to do the same
> > thing. (That is, if I understand correctly what translate_pid does.)
> 
> I'm not sure what you think translate_pid() is doing since you're not
> saying what you think it does.
> Examples from the old patchset:
> translate_pid(pid, ns, -1)      - get pid in our pid namespace
> translate_pid(pid, -1, ns)      - get pid in other pid namespace
> translate_pid(1, ns, -1)        - get pid of init task for namespace
> translate_pid(pid, -1, ns) > 0  - is pid is reachable from ns?
> translate_pid(1, ns1, ns2) > 0  - is ns1 inside ns2?
> translate_pid(1, ns1, ns2) == 0 - is ns1 outside ns2?
> translate_pid(1, ns1, ns2) == 1 - is ns1 equal ns2?
> 
> Allowing this syscall to yield pidfds as proper regular file fds and
> also taking pidfds as argument is an excellent way to kill a few
> problems at once:
> - cheap pid namespace introspection
> - creates a bridge between the "old" pid-based api and the "new" pidfd api

This second point would solve the problem of getting a new pidfd given a pid
indeed, without depending on /proc/<pid> at all. So kudos for that and I am
glad you are making it return pidfds (but correct me if I misunderstood what
you're planning to do with translate_fd). It also obviates any need for
dealing with procfs mount points.

> - allows us to get proper non-directory file descriptors for any pids we
>   like

Here I got a bit lost. AIUI pidfd is a directory fd. Why would we want it to
not be a directory fd? That would be ambigiuous with what pidfd_send_signal
expects.

Also would it be a bad idea to extend translate_pid to also do what we want
for the pidfd_wait syscall?  So translate_fd in this case would return an fd
that is just used for the pid's death status.

All of these extensions seem to mean translate_pid should probably take a
fourth parameter that tells it the target translation type?

They way I am hypothesizing, translate_pid, it should probably be
- translation to a pid in some ns
- translation of a pid to a pidfd
- translation of a pid to a "wait" fd which returns the death/reap process status.

If that makes sense, that would also avoid the need for a new syscall we are adding.

> The additional advantage is that people are already happy to add this
> syscall so simply extending it and routing it through the pidfd tree or
> Eric's tree is reasonable. (It should probably grow a flag argument. I
> need to start prototyping this.)

Great!

> > 
> > We still need a pidfd_clone() for atomicity reasons, but that's a
> > separate story. My goal is to be able to write a library that
> 
> Yes, on my todo list and I have a ported patch based on prior working
> rotting somehwere on my git server.

Is that different from using dup2 on a pidfd?  Sorry I don't follow what is
pidfd_clone / why it is needed.

thanks,

 - Joel