Re: [RFC] Storing cgroup id in page->private (Was: Re: [RFC] [PATCH 0/6] Provide cgroup isolation for buffered writes.)

[Andreas Dilger]

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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