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    SubjectRe: [PATCH] Re: [RFC PATCH] namespaces: fix leak on fork() failure
    On Fri, 2012-05-04 at 00:55 -0700, Eric W. Biederman wrote: 
    > Mike Galbraith <> writes:
    > > Namespaces have something in common with cgroups. synchronize_rcu()
    > > makes them somewhat less than wonderful for dynamic use.
    > Well unlike cgroups namespaces were not designed for heavy dynamic use.
    > Although it appears that vsftp puts them to that kind of use so some
    > of the design decisions are with revisiting.

    Yeah, the testcase was distilled from vsftp, so it must be beating on
    namespaces pretty hard to induce a bug report.

    > > default flags = SIGCHLD
    > >
    > > -namespace: flag |= CLONE_NEWPID
    > >
    > > marge:/usr/local/tmp/starvation # ./hackbench
    > > Running with 10*40 (== 400) tasks.
    > > Time: 2.636
    > > marge:/usr/local/tmp/starvation # ./hackbench -namespace
    > > Running with 10*40 (== 400) tasks.
    > > Time: 11.624
    > > marge:/usr/local/tmp/starvation # ./hackbench -namespace -all
    > > Running with 10*40 (== 400) tasks.
    > > Time: 51.474
    > CLONE_NEWUSER? I presume you have applied my latest user namespace
    > patches? Otherwise you are running completely half baked code.

    I was testing in mainline. While fiddling with the testcase and leakage
    monitor script, I decided to see what happens with all namespace flags.
    The others didn't cause any leakage, but did make things slow down.

    > hackbench? Which kernel are you running. Hackbench in some kernels is
    > really good at triggering cache ping-pong effects with pids, and creds.
    > So I'm not certain what to say there. In the latest kernels things
    > should be better with unix domain sockets as long as you don't actually
    > ask to pass your creds but hackbench is still a pretty ridiculous
    > benchmark. Oversharing is always going to be bad for performance.

    Hackbench was just to show the price of hefty namespace usage.

    > > You can create trash quickly, but you have to haul it away.
    > Well synchronize_rcu is much better in that respect than call_rcu, which
    > let's the trash build up but is never carried away.
    > The core design assumption with namespaces is that they will be used
    > much more than they will be created/destroyed, and as long as there are
    > progress guarantees in place I don't have a problem with that. At the
    > same time if there are easy things we can do to make things go faster
    > I am in favor of that notion.
    > Still especially in the case of hackbench I think it is worth asking the
    > question how much of the slow down is due to cache ping-pong due to
    > oversharing.

    Dunno, and doubt I'll have time to tinker with it more. Darn bugzilla
    thing keeps knocking on my mailbox with interesting bugs in places I
    know _diddly spit_ about.. like namespaces.


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