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

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Message-ID: <20110222130145.37cb151e@bike.lwn.net>
Date:	Tue, 22 Feb 2011 13:01:45 -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 Tue, 22 Feb 2011 18:12:54 +0100
Andrea Righi <arighi@...eler.com> wrote:

> The page_cgroup infrastructure, currently available only for the memory
> cgroup controller, can be used to store the owner of each page and
> opportunely track the writeback IO. This information is encoded in
> the upper 16-bits of the page_cgroup->flags.
> 
> A owner can be identified using a generic ID number and the following
> interfaces are provided to store a retrieve this information:
> 
>   unsigned long page_cgroup_get_owner(struct page *page);
>   int page_cgroup_set_owner(struct page *page, unsigned long id);
>   int page_cgroup_copy_owner(struct page *npage, struct page *opage);

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.

I'm tempted to say it might be better to just add a pointer to your
throtl_grp structure into struct page_cgroup.  Or maybe replace the
mem_cgroup pointer with a single pointer to struct css_set.  Both of
those ideas, though, probably just add unwanted extra overhead now to gain
generality which may or may not be wanted in the future.

jon
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