On 14.01.20 16:52, Kirill A. Shutemov wrote: > On Tue, Jan 14, 2020 at 11:49:19AM +0100, David Hildenbrand wrote: >> memmap_init_zone() is called for a physical memory region: pfn + size >> (nr_pages) >> >> The highest possible PFN you can have is "-1(unsigned long) >> >> PFN_SHIFT". So even if you would want to add the very last section, the >> PFN would still be smaller than -1UL << PFN_SECTION_SHIFT. > > PFN_SHIFT? I guess you mean PAGE_SHIFT. Yes :) > > Of course PFN can be more than -1UL >> PAGE_SHIFT. Like on 32-bit x86 with > PAE it is ((1ULL << 36) - 1) >> PAGE_SHIFT. That's the whole reason for > PAE. You are right about PAE, but I think you agree that is is a special case. > > The highest possible PFN must fit into phys_addr_t when shifted left by > PAGE_SHIFT and must fit into unsigned long. It's can be -1UL if > phys_addr_t is 64-bit. > Right, and for 32bit, that would mean (assuming something like 12bit PAGE_SHIFT) if you have -1 (0xffffffff) that the biggest possible address is 0xfffffffffff (44bit). In that case, the existing code would already break because "end_pfn" (is actually +1, pointing after the one to initialize), would overflow to 0 and you would have an endless loop in memmap_init_zone(). Now, after thischange you not only get an endless loop when trying to init the very last PFN, but when trying to init a PFN in the very last section (section_nr= -1 - e.g., the last 128MB). I don't think there is any sane use case where you initialize something partially in the last section that is possible with any hardware address extension mechanism. -- Thanks, David / dhildenb
