Re: [RFC PATCH] locks: Show only file_locks created in the same pidns as current process

Jeff Layton
2016-08-02T20:07:50Z

On Tue, 2016-08-02 at 15:44 -0400, J. Bruce Fields wrote:
> On Tue, Aug 02, 2016 at 02:09:22PM -0500, Eric W. Biederman wrote:
> >
> > > > "J. Bruce Fields" <bfields@fieldses.org> writes:
> >
> > >
> > > On Tue, Aug 02, 2016 at 11:00:39AM -0500, Eric W. Biederman wrote:
> > > >
> > > > > > > > Nikolay Borisov <kernel@kyup.com> writes:
> > > >
> > > > >
> > > > > Currently when /proc/locks is read it will show all the file locks
> > > > > which are currently created on the machine. On containers, hosted
> > > > > on busy servers this means that doing lsof can be very slow. I
> > > > > observed up to 5 seconds stalls reading 50k locks, while the container
> > > > > itself had only a small number of relevant entries. Fix it by
> > > > > filtering the locks listed by the pidns of the current process
> > > > > and the process which created the lock.
> > > >
> > > > The locks always confuse me so I am not 100% connecting locks
> > > > to a pid namespace is appropriate.
> > > >
> > > > That said if you are going to filter by pid namespace please use the pid
> > > > namespace of proc, not the pid namespace of the process reading the
> > > > file.
> > >
> > > Oh, that makes sense, thanks.
> > >
> > > What does /proc/mounts use, out of curiosity?  The mount namespace that
> > > /proc was originally mounted in?
> >
> > /proc/mounts -> /proc/self/mounts
>
> D'oh, I knew that.
>
> >
> > /proc/[pid]/mounts lists mounts from the mount namespace of the
> > appropriate process.
> >
> > That is another way to go but it is a tread carefully thing as changing
> > things that way it is easy to surprise apparmor or selinux rules and be
> > surprised you broke someones userspace in a way that prevents booting.
> > Although I suspect /proc/locks isn't too bad.
>
> OK, thanks.
>
> /proc/[pid]/locks might be confusing.  I'd expect it to be "all the
> locks owned by this task", rather than "all the locks owned by pid's in
> the same pid namespace", or whatever criterion we choose.
>
> Uh, I'm still trying to think of the Obviously Right solution here, and
> it's not coming.
>
> --b.

I'm a little leery of changing how this works. It has always been
maintained as a legacy interface, so do we run the risk of breaking
something if we turn it into a per-namespace thing? This also doesn't
solve the problem of slow traversal in the init_pid_ns -- only in a
container.

I also can't help but feel that /proc/locks is just showing its age. It
was fine in the late 90's, but its limitations are just becoming more
apparent as things get more complex. It was never designed for
performance as you end up thrashing several spinlocks when reading it.

Maybe it's time to think about presenting this info in another way? A
global view of all locks on the system is interesting but maybe it
would be better to present it more granularly somehow?

I guess I should go look at what lsof actually does with this info...

--
Jeff Layton <jlayton@poochiereds.net>