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 02:11:15PM -0500, Vivek Goyal wrote:
> On Thu, Mar 10, 2011 at 10:57:52AM -0800, Justin TerAvest wrote:
> > On Thu, Mar 10, 2011 at 10:15 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > > On Thu, Mar 10, 2011 at 10:08:03AM -0800, Justin TerAvest wrote:
> > >
> > > [..]
> > >> > I don't like to increase size of page_cgroup but I think you can record
> > >> > information without increasing size of page_cgroup.
> > >> >
> > >> > A) As Andrea did, encode it to pc->flags.
> > >> >   But I'm afraid that there is a racy case because memory cgroup uses some
> > >> >   test_and_set() bits.
> > >> > B) I wonder why the information cannot be recorded in page->private.
> > >> >   When page has buffers, you can record the information to buffer struct.
> > >> >   About swapio (if you take care of), you can record information to bio.
> > >>
> > >> Hi Kame,
> > >>
> > >> I'm concerned that by using something like buffer_heads stored in
> > >> page->private, we will only be supported on some filesystems and not
> > >> others. In addition, I'm not sure if all filesystems attach buffer
> > >> heads at the same time; if page->private is modified in the flusher
> > >> thread, we might not be able to determine the thread that dirtied the
> > >> page in the first place.
> > >
> > > 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. 

Thanks
Vivek