I don't think it will cause significant performance loss, because
the link between a page and a page_cgroup has already existed, which
the memory resource controller prepared. Bio_cgroup uses this as it is,
and does nothing about this.
And the link between page_cgroup and bio_cgroup isn't protected
by any additional spin-locks, since the associated bio_cgroup is
guaranteed to exist as long as the bio_cgroup owns pages.
I've just noticed that most of overhead comes from the spin-locks
when reclaiming the pages inside mem_cgroups and the spin-locks to
protect the links between pages and page_cgroups.
The latter overhead comes from the policy your team has chosen
that page_cgroup structures are allocated on demand. I still feel
this approach doesn't make any sense because linux kernel tries to
make use of most of the pages as far as it can, so most of them
have to be assigned its related page_cgroup. It would make us happy
if page_cgroups are allocated at the booting time.
I don't think there are any diffrences between a poiter and ID.
I think this ID is just a encoded version of the pointer.
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