Re: [RFC PATCH 0/5] mm, memory_hotplug: allocate memmap from hotadded memory

[Michal Hocko]

On Mon 31-07-17 19:58:30, Gerald Schaefer wrote:
> On Mon, 31 Jul 2017 17:53:50 +0200
> Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> 
> > On Mon 31-07-17 17:04:59, Gerald Schaefer wrote:
[...]
> > > Well, it still looks like we cannot do w/o splitting up add_memory():
> > > 1) (only) set up section map during our initial memory detection, w/o
> > > allocating struct pages, so that the sysfs entries get created also for
> > > our offline memory (or else we have no way to online it later)
> > > 2) set up vmemmap and allocate struct pages with your new altmap approach
> > > during our MEM_GOING_ONLINE callback, because only now the memory is really
> > > accessible
> > 
> > As I've tried to mentioned in my other response. This is not possible
> > because there are memory hotplug usecases which never do an explicit
> > online.
> 
> Of course the default behaviour should not change, we only need an option
> to do the "2-stage-approach". E.g. we would call __add_pages() from our
> MEM_GOING_ONLINE handler, and not from arch_add_memory() as before, but
> then we would need some way to add memory sections (for generating sysfs
> memory blocks) only, without allocating struct pages. See also below.

I would have to check that more deeply. I am not sure some parts of
memblock infrastructure depends on struct page existence.
 
> > I am sorry to ask again. But why exactly cannot we make the range
> > accessible from arch_add_memory on s390?
> 
> We have no acpi or other events to indicate new memory, both online and
> offline memory needs to be (hypervisor) defined upfront, and then we want
> to be able to use memory hotplug for ballooning during runtime.
> 
> Making the range accessible is equivalent to a hypervisor call that assigns
> the memory to the guest. The problem with arch_add_memory() is now that
> this gets called from add_memory(), which we call during initial memory
> detection for the offline memory ranges. At that time, assigning all
> offline memory to the guest, and thus making it accessible, would break
> the ballooning usecase (even if it is still offline in Linux, the
> hypervisor could not use it for other guests any more).

OK, I guess I see your point. Thanks for the clarification. I will try
to think about this limitation but I will rule simply disable the
feature for the initial inclusion. s390 can be done later.

> The main difference to other architectures is that we can not simply
> call add_memory() (and thus arch_add_memory()) at the time when the
> offline memory is actually supposed to get online (e.g. triggered by acpi).
> We rather need to somehow make sure that the offline memory is detected
> early, and sysfs entries are created for it, so that it can be set online
> later on demand.
> 
> Maybe our design to use add_memory() for offline ranges during memory
> detection was wrong, or overkill, since we actually only need to establish
> a memory section, if I understood the sysfs code right. But I currently
> see no other way to make sure that we get the sysfs attributes. And of
> course the presence of users that work on offline struct pages, like
> valid_zones, is also not helpful.

Yeah, but I suspect that we can make the whole memory hotplug sysfs API
independant on memory sections and struct page states. I am adding this
to my todo list.
-- 
Michal Hocko
SUSE Labs