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

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Mar 10, 2011 at 02:24:07PM -0700, Andreas Dilger wrote:
> 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.

Andreas,

As Chris mentioned, will providing callbacks so that filesystem can
save/restore page->private be reasonable?

Thanks
Vivek