Re: [PATCH v1 1/2] mm/page_alloc.c: Don't set pages PageReserved() when offlining

[David Hildenbrand]

> > - and I remember that PG_reserved on memory holes is relevant to
> > detect MMIO pages. (e.g., looking at KVM code ...)
>
> I can see kvm_is_reserved_pfn() which checks both pfn_valid and
> PageReserved. How does this help to detect memory holes though?
> Any driver might be setting the page reserved.

See my other mail. This is mostly to not touch MMIO pages and 
ZONE_DEVICE pages ... well and /dev/mem mapped pages.

>   
> > > > > Also is the hole inside a hotplugable memory something we really have to
> > > > > care about. Has anybody actually seen a platform to require that?
> > > >
> > > > That's what I was asking. I can see "support" for this was added basically
> > > > right from the beginning. I'd say we rip that out and cleanup/simplify. I am
> > > > not aware of a platform that requires this. Especially, memory holes on
> > > > DIMMs (detected during boot) seem like an unlikely thing.
> > >
> > > The thing is that the hotplug development shows ad-hoc decisions
> > > throughout the code. It is even worse that it is hard to guess whether
> > > some hludges are a result of a careful design or ad-hoc trial and
> > > failure approach on setups that never were production. Building on top
> > > of that be preserving hacks is not going to improve the situation. So I
> > > am perfectly fine to focus on making the most straightforward setups
> > > work reliably. Even when there is a risk of breaking some odd setups. We
> > > can fix them up later but we would have at least a specific example and
> > > document it.
> >
> > Alright, I'll prepare a simple patch that rejects offlining memory with
>
> Is offlining an interesting path? I would expect onlining to be much
> more interesting one.

If you can't offline memory with holes, you can also not online memory 
with holes AFAIKS :)

Bootmem is online, and memory you can hotplug (initially offline) cannot 
have any holes.

--

Thanks,

David / dhildenb