On 2019/09/17 16:13, 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. Let me clear my thoughts. If I read correctly, the struct pages for sections (including partially present sections) on ZONE_DEVICE are allocated by vmemmap_populate(). And all the struct pages except (altmap->base_pfn) to (altmap->base_pfn + altmap->reserve) are initialized by memmap_init_zone() and memmap_init_zone_device(). Do struct pages for partially present sections go through a different process? Thanks, Toshiki Fukasawa > > 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. >
