Re: [PATCH] [23/31] HWPOISON: add memory cgroup filter

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Dec 10, 2009 at 10:21:13AM +0800, Balbir Singh wrote:
> * Andi Kleen <andi@firstfloor.org> [2009-12-10 02:42:12]:
> 
> > > While the functionality sounds useful, the interface (passing an inode
> > > number) feels a bit ugly to me. Also, if that group is deleted and a
> > > new cgroup created, you could end up reusing the inode number.
> > 
> > Please note this is just a testing interface, doesn't need to be
> > 100% fool-proof. It'll never be used in production.
> > 
> > > 
> > > How about an approach where you write either the cgroup path (relative
> > > to the memcg mount) or an fd open on the desired cgroup? Then you
> > > could store a (counted) css reference rather than an inode number,
> > > which would make the filter function cleaner too, since it would just
> > > need to compare css objects.
> > 
> > Sounds complicated, I assume it would be much more code?
> > I would prefer to keep the testing interfaces as simple as possible.
> >
> 
> We do this for cgroupstats and the code is not very complicated. In
> case you want to look, the user space code is at
> Documentation/accounting/getdelays.c and the kernel code is in
> kernel/taskstats.c 

Balbir, thanks for the tip.

We could keep an fd open on the desired cgroup, in user space: 

        #!/bin/bash

        mkdir /cgroup/hwpoison && \
        exec 9<>/cgroup/hwpoison/tasks || exit 1

A bit simpler than an in-kernel fget_light() or CSS refcount :)

Thanks,
Fengguang

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