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

[Vivek Goyal]

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@xxxxxxxxxx> 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.

> 
> I don't know that it's right to add the burden to copy the original
> value to everything that wants to use page->private.
> 

How many such places are there?

Thanks
Vivek
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