Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page

[Shakeel Butt]

+Cc Roman

On Fri, Feb 5, 2021 at 2:49 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
[snip]
> > > > Also, css_get is enough because page
> > > > has a reference to the memcg.
> > >
> > > tryget used to be there to guard against offlined memcg but we have
> > > concluded this is impossible in this path. tryget stayed there to catch
> > > some unexpected cases IIRC.
> >
> > Yeah, it can catch some unexpected cases. But why is this path
> > special so that we need a tryget?
>
> I do not remember details and the changelog of that change is not
> explicit but I suspect it was just because this one could trigger as
> there are external callers to memcg. Is this protection needed? I am not
> sure, this is for you to justify if you want to remove it.
>

It used to be css_tryget_online() which was changed to css_tryget()
and from the discussion at [1], it seemed css_get() would be enough
but we took a safer route.

Anyways, I think we can either take the page_memcg_rcu() route or put
explicit restrictions with page lock or lock_page_memcg() to guarantee
page and memcg binding. I don't have a strong opinion either way but I
think removing restrictions in future for new use-cases will be much
harder, so, page_memcg_rcu() approach seems more appropriate at least
for now.

[1] https://lore.kernel.org/linux-mm/CALvZod5pAv=u8L2Tgk0hDY7XAiiF2dvjC1omQ5BSfzFu_2zSXA@xxxxxxxxxxxxxx/