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

[David Hildenbrand]

On 22.10.19 10:20, Michal Hocko wrote:
> On Mon 21-10-19 17:54:35, David Hildenbrand wrote:
> > On 21.10.19 17:47, Michal Hocko wrote:
> > > On Mon 21-10-19 17:39:36, David Hildenbrand wrote:
> > > > On 21.10.19 16:43, Michal Hocko wrote:
> > > [...]
> > > > > We still set PageReserved before onlining pages and that one should be
> > > > > good to go as well (memmap_init_zone).
> > > > > Thanks!
> > > >
> > > > memmap_init_zone() is called when onlining memory. There, set all pages to
> > > > reserved right now (on context == MEMMAP_HOTPLUG). We clear PG_reserved when
> > > > onlining a page to the buddy (e.g., generic_online_page). If we would online
> > > > a memory block with holes, we would want to keep all such pages
> > > > (!pfn_valid()) set to reserved. Also, there might be other side effects.
> > >
> > > Isn't it sufficient to have those pages in a poisoned state? They are
> > > not onlined so their state is basically undefined anyway. I do not see
> > > how PageReserved makes this any better.
> >
> > It is what people have been using for a long time. Memory hole ->
> > PG_reserved. The memmap is valid, but people want to tell "this here is
> > crap, don't look at it".
>
> The page is poisoned, right? If yes then setting the reserved bit
> doesn't make any sense.

No it's not poisoned AFAIK. It should be initialized - and I remember 
that PG_reserved on memory holes is relevant to detect MMIO pages. 
(e.g., looking at KVM code ...)

> > > 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 
memory holes. We can apply that and see if anybody screams out loud. If 
not, we can clean up that crap.

--

Thanks,

David / dhildenb