IKEDA, Munehiro wrote:
> Cgroup's memory controller has a control file "memory.force_empty"
> to reset usage account charged to a cgroup. The account shouldn't
> be reset if one or more processes are attached to the cgroup (at
> least for memory controller, IMHO). So mem_cgroup_force_empty()
> is implemented to return -EBUSY and do nothing if so.
> However, cgroup on hierarchy root faultily might be a exception.
> Even if processes are attached to root cgroup (which is a "default"
> cgroup for processes), forcing-empty can run by writing something to
> memory.force_empty and it'll never end.
>
I found this bug last week, and I've made patches to fix it, but then
I was on vacation. I'll send the patches out soon.
> Following patch prevents this issue.
>
> This patch is for cgroup infrastructure code. The issue can be
> measured by modifying memory controller code also, namely to change
> mem_cgroup_force_empty() to see CSS_ROOT bit of css->flags.
> I believe cgroup->count approach like the patch below is rather
> generic and reasonable, how does that sound?
>
It's ok for the top_group's count to be 0 due to the top_cgroup hack.
With this patch, the top cgroup's count will be always >0, even if it
has no tasks in it, so writing to top_cgroup's force_empty will always
return -EBUSY.
> Paul, Balbir?
>
>
>
> Signed-off-by: Munehiro "Muuhh" Ikeda <m-ikeda@xxxxxxxxxxxxx>
>
> diff -uNrp linux-2.6.27-rc3.orig/kernel/cgroup.c linux-2.6.27-rc3/kernel/cgroup.c
> --- linux-2.6.27-rc3.orig/kernel/cgroup.c 2008-08-12 21:55:39.000000000 -0400
> +++ linux-2.6.27-rc3/kernel/cgroup.c 2008-08-15 20:52:52.000000000 -0400
> @@ -2264,8 +2264,10 @@ static void init_cgroup_css(struct cgrou
> css->cgroup = cgrp;
> atomic_set(&css->refcnt, 0);
> css->flags = 0;
> - if (cgrp == dummytop)
> + if (cgrp == dummytop) {
> set_bit(CSS_ROOT, &css->flags);
> + atomic_set(&css->cgroup->count, 1);
> + }
> BUG_ON(cgrp->subsys[ss->subsys_id]);
> cgrp->subsys[ss->subsys_id] = css;
> }
>
>
>
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