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

[Michal Hocko]

On Tue 22-10-19 10:23:37, David Hildenbrand wrote:
> 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

Dohh, it seems I still keep confusing myself. You are right the page is
initialized at this stage. A potential hole in RAM or ZONE_DEVICE memory
will just not hit the page allocator. Sorry about the noise.

> - 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.
 
> > > > 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.

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