On Tue, 22 Feb 2011 18:12:54 +0100
Andrea Righi [off-list ref] 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

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