On 14.01.20 17:50, David Hildenbrand wrote: > 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(). Correction: If end_pfn overflows to 0, you would get no loop iteration at all. -- Thanks, David / dhildenb
