From: Joel Fernandes <joel@joelfernandes.org>
To: Christian Brauner <christian@brauner.io>
Cc: "Daniel Colascione" <dancol@google.com>,
"Suren Baghdasaryan" <surenb@google.com>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Sultan Alsawaf" <sultan@kerneltoast.com>,
"Tim Murray" <timmurray@google.com>,
"Michal Hocko" <mhocko@kernel.org>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Arve Hjønnevåg" <arve@android.com>,
"Todd Kjos" <tkjos@android.com>,
"Martijn Coenen" <maco@android.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Peter Zijlstra" <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
"open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
linux-mm <linux-mm@kvack.org>,
kernel-team <kernel-team@android.com>,
"Oleg Nesterov" <oleg@redhat.com>,
"Andy Lutomirski" <luto@amacapital.net>,
"Serge E. Hallyn" <serge@hallyn.com>,
"Kees Cook" <keescook@chromium.org>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
Date: Tue, 19 Mar 2019 21:52:49 -0400 [thread overview]
Message-ID: <20190320015249.GC129907@google.com> (raw)
In-Reply-To: <20190319231020.tdcttojlbmx57gke@brauner.io>
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
next prev parent reply other threads:[~2019-03-20 1:52 UTC|newest]
Thread overview: 105+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-10 20:34 [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android Sultan Alsawaf
2019-03-10 21:03 ` Greg Kroah-Hartman
2019-03-10 21:26 ` Sultan Alsawaf
2019-03-11 16:32 ` Joel Fernandes
2019-03-11 16:37 ` Joel Fernandes
2019-03-11 17:43 ` Michal Hocko
2019-03-11 17:58 ` Sultan Alsawaf
2019-03-11 20:10 ` Suren Baghdasaryan
2019-03-11 20:46 ` Sultan Alsawaf
2019-03-11 21:11 ` Joel Fernandes
2019-03-11 21:46 ` Sultan Alsawaf
2019-03-11 22:15 ` Suren Baghdasaryan
2019-03-11 22:36 ` Sultan Alsawaf
2019-03-12 8:05 ` Michal Hocko
2019-03-12 14:36 ` Suren Baghdasaryan
2019-03-12 15:25 ` Matthew Wilcox
2019-03-12 15:33 ` Michal Hocko
2019-03-12 15:39 ` Michal Hocko
2019-03-12 16:37 ` Sultan Alsawaf
2019-03-12 16:48 ` Michal Hocko
2019-03-12 16:58 ` Michal Hocko
2019-03-12 17:15 ` Suren Baghdasaryan
2019-03-12 17:17 ` Tim Murray
2019-03-12 17:45 ` Sultan Alsawaf
2019-03-12 18:43 ` Tim Murray
2019-03-12 18:50 ` Christian Brauner
2019-03-14 17:47 ` Joel Fernandes
2019-03-14 20:49 ` Sultan Alsawaf
2019-03-15 2:54 ` Joel Fernandes
2019-03-15 3:43 ` Sultan Alsawaf
2019-03-15 3:16 ` Steven Rostedt
2019-03-15 3:45 ` Sultan Alsawaf
2019-03-15 4:36 ` Daniel Colascione
2019-03-15 13:36 ` Joel Fernandes
2019-03-15 15:56 ` Suren Baghdasaryan
2019-03-15 16:12 ` Daniel Colascione
2019-03-15 16:43 ` Steven Rostedt
2019-03-15 17:17 ` Daniel Colascione
2019-03-15 18:03 ` Christian Brauner
2019-03-15 18:13 ` Joel Fernandes
2019-03-15 18:24 ` Christian Brauner
2019-03-15 18:49 ` Joel Fernandes
2019-03-16 17:31 ` Suren Baghdasaryan
2019-03-16 18:00 ` Daniel Colascione
2019-03-16 18:57 ` Christian Brauner
2019-03-16 19:37 ` Suren Baghdasaryan
2019-03-17 1:53 ` Joel Fernandes
2019-03-17 11:42 ` Christian Brauner
2019-03-17 15:40 ` Daniel Colascione
2019-03-18 0:29 ` Christian Brauner
2019-03-18 23:50 ` Joel Fernandes
2019-03-19 22:14 ` Christian Brauner
2019-03-19 22:26 ` Joel Fernandes
2019-03-19 22:48 ` Daniel Colascione
2019-03-19 23:10 ` Christian Brauner
2019-03-20 1:52 ` Joel Fernandes [this message]
2019-03-20 2:42 ` pidfd design Daniel Colascione
2019-03-20 3:59 ` Christian Brauner
2019-03-20 7:02 ` Daniel Colascione
2019-03-20 11:33 ` Joel Fernandes
2019-03-20 18:26 ` Christian Brauner
2019-03-20 18:38 ` Daniel Colascione
2019-03-20 18:51 ` Christian Brauner
2019-03-20 18:58 ` Andy Lutomirski
2019-03-20 19:14 ` Christian Brauner
2019-03-20 19:40 ` Daniel Colascione
2019-03-21 17:02 ` Andy Lutomirski
2019-03-20 19:19 ` Joel Fernandes
2019-03-20 19:29 ` Daniel Colascione
2019-03-24 14:44 ` Serge E. Hallyn
2019-03-24 18:48 ` Joel Fernandes
2019-03-20 19:11 ` Joel Fernandes
2019-05-07 2:16 ` [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android Sultan Alsawaf
2019-05-07 7:04 ` Greg Kroah-Hartman
2019-05-07 7:27 ` Sultan Alsawaf
2019-05-07 7:43 ` Greg Kroah-Hartman
2019-05-07 8:12 ` Sultan Alsawaf
2019-05-07 10:58 ` Christian Brauner
2019-05-07 16:28 ` Suren Baghdasaryan
2019-05-07 16:38 ` Christian Brauner
2019-05-07 16:53 ` Sultan Alsawaf
2019-05-07 20:01 ` Suren Baghdasaryan
2019-05-07 18:46 ` Joel Fernandes
2019-05-07 17:17 ` Sultan Alsawaf
2019-05-07 17:29 ` Greg Kroah-Hartman
2019-05-07 11:09 ` Greg Kroah-Hartman
2019-05-07 12:26 ` Michal Hocko
2019-05-07 15:31 ` Oleg Nesterov
2019-05-07 16:35 ` Sultan Alsawaf
2019-05-09 15:56 ` Oleg Nesterov
2019-05-09 18:33 ` Sultan Alsawaf
2019-05-10 15:10 ` Oleg Nesterov
2019-05-13 16:45 ` Sultan Alsawaf
2019-05-14 16:44 ` Steven Rostedt
2019-05-14 17:31 ` Sultan Alsawaf
2019-05-15 14:58 ` Oleg Nesterov
2019-05-15 17:27 ` Sultan Alsawaf
2019-05-15 18:32 ` Steven Rostedt
2019-05-15 18:52 ` Sultan Alsawaf
2019-05-15 20:09 ` Steven Rostedt
2019-05-16 13:54 ` Oleg Nesterov
2019-03-17 16:35 ` Serge E. Hallyn
2019-03-17 17:11 ` Daniel Colascione
2019-03-17 17:16 ` Serge E. Hallyn
2019-03-17 22:02 ` Suren Baghdasaryan
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