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
