On Fri, Feb 5, 2021 at 6:32 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > 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. Yeah, there is no comment requiring that. I have seen all the callers of the alloc_page_buffers. I found that it is under page lock. But it seems it is not the key point here. I should delete those comments from the commit log. > > > > > 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. I am sure it is not needed. > > > > > 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. OK. > -- > Michal Hocko > SUSE Labs
