linux-kernel - Re: [PATCH 3/5] page_cgroup: make page tracking available for blkio

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Message-ID: <20110222162729.054fe596@bike.lwn.net>
Date:	Tue, 22 Feb 2011 16:27:29 -0700
From:	Jonathan Corbet <corbet@....net>
To:	Andrea Righi <arighi@...eler.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Ryo Tsuruta <ryov@...inux.co.jp>,
	Hirokazu Takahashi <taka@...inux.co.jp>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] page_cgroup: make page tracking available for blkio

On Wed, 23 Feb 2011 00:01:47 +0100
Andrea Righi <arighi@...eler.com> wrote:

> > My immediate observation is that you're not really tracking the "owner"
> > here - you're tracking an opaque 16-bit token known only to the block
> > controller in a field which - if changed by anybody other than the block
> > controller - will lead to mayhem in the block controller.  I think it
> > might be clearer - and safer - to say "blkcg" or some such instead of
> > "owner" here.
> 
> Basically the idea here was to be as generic as possible and make this
> feature potentially available also to other subsystems, so that cgroup
> subsystems may represent whatever they want with the 16-bit token.
> However, no more than a single subsystem may be able to use this feature
> at the same time.

That makes me nervous; it can't really be used that way unless we want to
say that certain controllers are fundamentally incompatible and can't be
allowed to play together.  For whatever my $0.02 are worth (given the
state of the US dollar, that's not a whole lot), I'd suggest keeping the
current mechanism, but make it clear that it belongs to your controller.
If and when another controller comes along with a need for similar
functionality, somebody can worry about making it more general.

jon
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