On Fri 13-04-18 15:07:14, Kirill Tkhai wrote: > On 13.04.2018 14:54, Michal Hocko wrote: > > 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. > > We definitely can move id allocation to mem_cgroup_css_alloc(), but this > is really not for an easy fix, which will be backported to stable. > > Moving idr destroy to mem_cgroup_free() hides IDR trick. My IMHO it's less > readable for a reader. > > The main problem is allocation asymmetric, and we shouldn't handle it on free path... Well, this is probably a matter of taste. I will not argue. I will not object if Johannes is OK with your patch. But the whole thing confused hell out of me so I would rather un-clutter it... -- Michal Hocko SUSE Labs
