linux-kernel - Re: [RFC][PATCH 0/3] cgroups: support for module-loadable subsystems

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Message-ID: <4AEF8F7E.1000007@cn.fujitsu.com>
Date:	Tue, 03 Nov 2009 10:03:42 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	bblum@...rew.cmu.edu
CC:	linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 0/3] cgroups: support for module-loadable subsystems

Ben Blum wrote:
> This patch series implements simple support for building and loading
> subsystems as modules, both within and outside the kernel source tree.
> Module unloading is as yet unimplemented - it will require more advanced
> reference counting in the mount/unmount code, and I plan to work on this
> soon.
> 

I doubt the value of module-loadable subsystems. A cgroup subsystem
is usually a kernel resource controller, and normally it needs to
add some hooks in some in-kernel structures and functions, which
makes it impossible to be a module.

In fact, net_cls is the only subsystem that can be a module, and
make it a module doesn't seem to have real benefit?

> Patch #1 sets up the subsys[] array so its contents can be dynamic as
> modules appear and (eventually) disappear. I introduce an rwsem called
> subsys_mutex to protect against concurrent loads/unloads/reads, and
> modify iterations over the array to handle when subsystems are absent.
> 
> Patch #2 implements an interface for modules to load subsystems, called
> cgroup_load_subsys, similar to cgroup_init_subsys, and adds a module
> pointer in struct cgroup_subsys.
> 
> Patch #3 modifies the net_cls subsystem, which already had some module
> declarations, to be configurable as a module, which also serves as a
> simple proof-of-concept.
> 
> -- bblum
>  
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