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

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Vivek Goyal's message of 2011-03-10 16:38:32 -0500:
> 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?

Just to clarify, I think saving/restoring page->private is going to be
hard.  I'd rather just have a call back that says here's a page, storage
this for the block io controller please, and another one that returns
any previously stored info.

-chris