On Thu 22-11-18 16:26:40, David Hildenbrand wrote: > On 22.11.18 11:12, Wei Yang wrote: > > During online_pages phase, pgdat->nr_zones will be updated in case this > > zone is empty. > > > > Currently the online_pages phase is protected by the global lock > > mem_hotplug_begin(), which ensures there is no contention during the > > update of nr_zones. But this global lock introduces scalability issues. > > > > This patch is a preparation for removing the global lock during > > online_pages phase. Also this patch changes the documentation of > > node_size_lock to include the protectioin of nr_zones. > > I looked into locking recently, and there is more to it. > > Please read: > > commit dee6da22efac451d361f5224a60be2796d847b51 > Author: David Hildenbrand <david@xxxxxxxxxx> > Date: Tue Oct 30 15:10:44 2018 -0700 > > memory-hotplug.rst: add some details about locking internals > > Let's document the magic a bit, especially why device_hotplug_lock is > required when adding/removing memory and how it all play together with > requests to online/offline memory from user space. > > Short summary: Onlining/offlining of memory requires the device_hotplug_lock > as of now. Well, I would tend to disagree here. You might be describing the current state of art but the device_hotplug_lock doesn't make much sense for the memory hotplug in principle. There is absolutely nothing in the core MM that would require this lock. The current state just uses a BKL in some sense and we really want to get rid of that longterm. This patch is a tiny step in that direction and I suspect many more will need to come on the way. We really want to end up with a clear scope of each lock being taken. A project for a brave soul... -- Michal Hocko SUSE Labs
