On 9/17/19 12:21 PM, Qian Cai wrote: > On Tue, 2019-09-17 at 11:49 -0400, Waiman Long wrote: >> On 9/17/19 3:13 AM, David Hildenbrand wrote: >>> On 17.09.19 04:34, Toshiki Fukasawa wrote: >>>> On 2019/09/09 16:46, David Hildenbrand wrote: >>>>> Let's take a step back here to understand the issues I am aware of. I >>>>> think we should solve this for good now: >>>>> >>>>> A PFN walker takes a look at a random PFN at a random point in time. It >>>>> finds a PFN with SECTION_MARKED_PRESENT && !SECTION_IS_ONLINE. The >>>>> options are: >>>>> >>>>> 1. It is buddy memory (add_memory()) that has not been online yet. The >>>>> memmap contains garbage. Don't access. >>>>> >>>>> 2. It is ZONE_DEVICE memory with a valid memmap. Access it. >>>>> >>>>> 3. It is ZONE_DEVICE memory with an invalid memmap, because the section >>>>> is only partially present: E.g., device starts at offset 64MB within a >>>>> section or the device ends at offset 64MB within a section. Don't access it. >>>> I don't agree with case #3. In the case, struct page area is not allocated on >>>> ZONE_DEVICE, but is allocated on system memory. So I think we can access the >>>> struct pages. What do you mean "invalid memmap"? >>> No, that's not the case. There is no memory, especially not system >>> memory. We only allow partially present sections (sub-section memory >>> hotplug) for ZONE_DEVICE. >>> >>> invalid memmap == memmap was not initialized == struct pages contains >>> garbage. There is a memmap, but accessing it (e.g., pfn_to_nid()) will >>> trigger a BUG. >>> >> As long as the page structures exist, they should be initialized to some >> known state. We could set PagePoison for those invalid memmap. It is the > Sounds like you want to run page_init_poison() by default. Yes for those pages that are not initialized otherwise. I don't want to run page_init_poison() for the whole ZONE_DEVICE memory range as it can take a while if we are talking about TBs of persistent memory. Also most of the pages will be reinitialized anyway in the init process. So it is mostly a wasted effort. However, for those reserved pages that are not being exported to the memory management layer, having them initialized to a known state will cause less problem down the road. Cheers, Longman
