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Date: Fri, 17 Apr 2009 11:24:33 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Andrea Righi <righi.andrea@gmail.com>, Paul Menage <menage@google.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
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Subject: Re: [PATCH 3/9] bio-cgroup controller
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On Fri, 17 Apr 2009 10:49:43 +0900
Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> wrote:

> Hi,
> 
> I have a few question.
>    - I have not yet fully understood how your controller are using
>      bio_cgroup. If my view is wrong please tell me.
> 
> o In my view, bio_cgroup's implementation strongly depends on
>    page_cgoup's. Could you explain for what purpose does this
>    functionality itself should be implemented as cgroup subsystem?
>    Using page_cgoup and implementing tracking APIs is not enough?

I'll definitely do "Nack" to add full bio-cgroup members to page_cgroup.
Now, page_cgroup is 40bytes(in 64bit arch.) And all of them are allocated at
boot time as memmap. (and add member to struct page is much harder ;)

IIUC, feature for "tracking bio" is just necesary for pages for I/O.
So, I think it's much better to add misc. information to struct bio not to the page.
But, if people want to add "small hint" to struct page or struct page_cgroup
for tracking buffered I/O, I'll give you help as much as I can.
Maybe using "unused bits" in page_cgroup->flags is a choice with no overhead.

Thanks,
-Kame