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
