On Fri 13-04-18 14:49:32, Kirill Tkhai wrote: > On 13.04.2018 14:38, Michal Hocko wrote: > > On Fri 13-04-18 14:29:11, Kirill Tkhai wrote: [...] > >> mem_cgroup_id_put_many() unpins css, but this may be not the last reference to the css. > >> Thus, we release ID earlier, then all references to css are freed. > > > > Right and so what. If we have released the idr then we are not going to > > do that again in css_free. That is why we have that memcg->id.id > 0 > > check before idr_remove and memcg->id.id = 0 for the last memcg ref. > > count. So again, why cannot we do the clean up in mem_cgroup_free and > > have a less confusing code? Or am I just not getting your point and > > being dense here? > > We can, but mem_cgroup_free() called from mem_cgroup_css_alloc() is unlikely case. > The likely case is mem_cgroup_free() is called from mem_cgroup_css_free(), where > this idr manipulations will be a noop. Noop in likely case looks more confusing > for me. Well, I would really prefer to have _free being symmetric to _alloc so that you can rely that the full state is gone after _free is called. This confused the hell out of me. Because I _did_ expect that mem_cgroup_free would do that and so I was looking at completely different place. > Less confusing will be to move > > memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL, > 1, MEM_CGROUP_ID_MAX, > GFP_KERNEL); > > into mem_cgroup_css_alloc(). How are you think about this? I would have to double check. Maybe it can be done on top. But for the actual fix and a stable backport potentially should be as clear as possible. Your original patch would be just fine but if I would prefer mem_cgroup_free for the symmetry. -- Michal Hocko SUSE Labs
