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

[Michal Hocko]

On Fri 05-02-21 17:14:30, Muchun Song wrote:
> On Fri, Feb 5, 2021 at 4:36 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Fri 05-02-21 14:27:19, Muchun Song wrote:
> > > The get_mem_cgroup_from_page() is called under page lock, so the page
> > > memcg cannot be changed under us.
> >
> > Where is the page lock enforced?
> 
> Because it is called from alloc_page_buffers(). This path is under
> page lock.

I do not see any page lock enforecement there. There is not even a
comment requiring that. Can we grow more users where this is not the
case? There is no actual relation between alloc_page_buffers and
get_mem_cgroup_from_page except that the former is the only _current_
existing user. I would be careful to dictate locking based solely on
that.

> > > 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.

> > > If we really want to make the get_mem_cgroup_from_page() suitable for
> > > arbitrary page, we should use page_memcg_rcu() instead of page_memcg()
> > > and call it after rcu_read_lock().
> >
> > What is the primary motivation to change this code? is the overhead of
> > tryget/RCU something that needs optimizing?
> 
> Actually, the rcu_read_lock() is not necessary here. So it is better to
> remove it (indeed reduce some code).

Please state your reasoning in the changelog including benefits we get
from it.
-- 
Michal Hocko
SUSE Labs