On 17.09.19 11:32, Toshiki Fukasawa wrote: > 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? No. However, the memmap is initialized via move_pfn_range_to_zone(). So partially present sections will have partially uninitialized memmaps. But I get your point. I just saw that pfn_valid() does take care of the subsection map via pfn_section_valid(). - pfn_present() does not, which is weird, but ok. So I agree, in case #3 might have a partially uninitialized memmap, but we can test via pfn_valid() if the memory is at least valid. So there is some way to test. Then we're back to the race between adding the memory and initializing the memmap. Thanks! > > Thanks, > Toshiki Fukasawa >> -- Thanks, David / dhildenb
