Re: [PATCH v2 0/8] mm/memory_hotplug: Factor out memory block device handling

[Dan Williams <dan.j.williams@intel.com>]

On Tue, May 7, 2019 at 11:38 AM David Hildenbrand <david@redhat.com> wrote:
>
> We only want memory block devices for memory to be onlined/offlined
> (add/remove from the buddy). This is required so user space can
> online/offline memory and kdump gets notified about newly onlined memory.
>
> Only such memory has the requirement of having to span whole memory blocks.
> Let's factor out creation/removal of memory block devices. This helps
> to further cleanup arch_add_memory/arch_remove_memory() and to make
> implementation of new features easier. E.g. supplying a driver for
> memory block devices becomes way easier (so user space is able to
> distinguish different types of added memory to properly online it).
>
> Patch 1 makes sure the memory block size granularity is always respected.
> Patch 2 implements arch_remove_memory() on s390x. Patch 3 prepares
> arch_remove_memory() to be also called without CONFIG_MEMORY_HOTREMOVE.
> Patch 4,5 and 6 factor out creation/removal of memory block devices.
> Patch 7 gets rid of some unlikely errors that could have happened, not
> removing links between memory block devices and nodes, previously brought
> up by Oscar.
>
> Did a quick sanity test with DIMM plug/unplug, making sure all devices
> and sysfs links properly get added/removed. Compile tested on s390x and
> x86-64.
>
> Based on git://git.cmpxchg.org/linux-mmots.git
>
> Next refactoring on my list will be making sure that remove_memory()
> will never deal with zones / access "struct pages". Any kind of zone
> handling will have to be done when offlining system memory / before
> removing device memory. I am thinking about remove_pfn_range_from_zone()",
> du undo everything "move_pfn_range_to_zone()" did.
>
> v1 -> v2:
> - s390x/mm: Implement arch_remove_memory()
> -- remove mapping after "__remove_pages"
>
>
> David Hildenbrand (8):
>   mm/memory_hotplug: Simplify and fix check_hotplug_memory_range()
>   s390x/mm: Implement arch_remove_memory()
>   mm/memory_hotplug: arch_remove_memory() and __remove_pages() with
>     CONFIG_MEMORY_HOTPLUG
>   mm/memory_hotplug: Create memory block devices after arch_add_memory()
>   mm/memory_hotplug: Drop MHP_MEMBLOCK_API

So at a minimum we need a bit of patch staging guidance because this
obviously collides with the subsection bits that are built on top of
the existence of MHP_MEMBLOCK_API. What trigger do you envision as a
replacement that arch_add_memory() use to determine that subsection
operations should be disallowed?