Subject: Re: [RFC] Storing cgroup id in page->private (Was: Re: [RFC] [PATCH 0/6] Provide cgroup isolation for buffered writes.)
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From: Andreas Dilger <adilger@dilger.ca>
In-Reply-To: <1299791640-sup-1874@think>
Date: Thu, 10 Mar 2011 14:24:07 -0700
Cc: Vivek Goyal <vgoyal@redhat.com>, Justin TerAvest <teravest@google.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        m-ikeda <m-ikeda@ds.jp.nec.com>, jaxboe <jaxboe@fusionio.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, ryov <ryov@valinux.co.jp>,
        taka <taka@valinux.co.jp>, "righi.andrea" <righi.andrea@gmail.com>,
        guijianfeng <guijianfeng@cn.fujitsu.com>,
        balbir <balbir@linux.vnet.ibm.com>, ctalbott <ctalbott@google.com>,
        nauman <nauman@google.com>, mrubin <mrubin@google.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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To: Chris Mason <chris.mason@oracle.com>
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On 2011-03-10, at 2:15 PM, Chris Mason wrote:
> Excerpts from Vivek Goyal's message of 2011-03-10 14:41:06 -0500:
>> On Thu, Mar 10, 2011 at 02:11:15PM -0500, Vivek Goyal wrote:
>>>>> I think the person who dirtied the page can store the information in
>>>>> page->private (assuming buffer heads were not generated) and if flusher
>>>>> thread later ends up generating buffer heads and ends up modifying
>>>>> page->private, this can be copied in buffer heads?
>>>> 
>>>> This scares me a bit.
>>>> 
>>>> As I understand it, fs/ code expects total ownership of page->private.
>>>> This adds a responsibility for every user to copy the data through and
>>>> store it in the buffer head (or anything else). btrfs seems to do
>>>> something entirely different in some cases and store a different kind
>>>> of value.
>>> 
>>> If filesystems are using page->private for some other purpose also, then
>>> I guess we have issues. 
>>> 
>>> I am ccing linux-fsdevel to have some feedback on the idea of trying
>>> to store cgroup id of page dirtying thread in page->private and/or buffer
>>> head for tracking which group originally dirtied the page in IO controller
>>> during writeback.
>> 
>> A quick "grep" showed that btrfs, ceph and logfs are using page->private
>> for other purposes also.
>> 
>> I was under the impression that either page->private is null or it 
>> points to buffer heads for the writeback case. So storing the info
>> directly in either buffer head directly or first in page->private and
>> then transferring it to buffer heads would have helped. 
> 
> Right, btrfs has its own uses for page->private, and we expect to own
> it.  With a proper callback, the FS could store the extra information you
> need in out own structs.

There is no requirement that page->private ever points to a buffer_head, and Lustre clients use it for its own tracking structure (never touching buffer_heads at all).  Any assumption about what a filesystem is storing in page->private in other parts of the code is just broken.

Cheers, Andreas


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