Re: [PATCH 3/5] page_cgroup: make page tracking available for blkio

Jonathan Corbet
2011-02-22T20:01:50Z

On Tue, 22 Feb 2011 18:12:54 +0100
Andrea Righi <arighi@develer.com> wrote:

> The page_cgroup infrastructure, currently available only for the memory
> cgroup controller, can be used to store the owner of each page and
> opportunely track the writeback IO. This information is encoded in
> the upper 16-bits of the page_cgroup->flags.
>
> A owner can be identified using a generic ID number and the following
> interfaces are provided to store a retrieve this information:
>
> unsigned long page_cgroup_get_owner(struct page *page);
> int page_cgroup_set_owner(struct page *page, unsigned long id);
> int page_cgroup_copy_owner(struct page *npage, struct page *opage);

My immediate observation is that you're not really tracking the "owner"
here - you're tracking an opaque 16-bit token known only to the block
controller in a field which - if changed by anybody other than the block
controller - will lead to mayhem in the block controller. I think it
might be clearer - and safer - to say "blkcg" or some such instead of
"owner" here.

I'm tempted to say it might be better to just add a pointer to your
throtl_grp structure into struct page_cgroup. Or maybe replace the
mem_cgroup pointer with a single pointer to struct css_set. Both of
those ideas, though, probably just add unwanted extra overhead now to gain
generality which may or may not be wanted in the future.

jon