Re: [PATCH v2] mm, hotplug: move init_currently_empty_zone() under zone_span_lock protection

[Michal Hocko <mhocko@suse.com>]

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@redhat.com>
> 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