This only happens with kernels having initramfs builtin. W.R.T following memory map, first kernel ends up in creating per-cpu crash notes (ELF PT_NOTE) in the higher memory area. When a initramfs image is used, most often the crash_note ends up in "pfn's" belonging to 0x2000.0000 When crashkernel tries to access the PT_NOTEs (crash_notes per cpu variable) via __pfn_to_section <= pfn_to_page <= kmap, we end up in crash, as the sections belong to pfn 0x20006c00 (mem_section[2][0]) for a crash kernel is not populated. Creating new mappings using ioremap solves the problem, which is pretty much what the patch does. Determined physical RAM map: memory: 0000000004eff000 @ 0000000000100000 (usable) memory: 000000000a000000 @ 0000000005000000 (usable) <<<< Crashkernel reserved area memory: 0000000030000000 @ 0000000020000000 (usable) ------------ mem_section[0][0]:0x8000000004180003 mem_section[1][0]:0x0 mem_section[2][0]:0x8000000003e00003 mem_section[3][0]:0x8000000003e00003 mem_section[4][0]:0x8000000003e00003 mem_section[5][0]:0x0 mem_section[6][0]:0x0 << Repeats rest of 1024 times >> get_crash_notes_per_cpu: crash_notes addr = 20006c00 <<32 entries>> get_crash_notes_per_cpu: crash_notes addr = 2013cc00 Crash Kernel ============ Determined physical RAM map: memory: 000000000a000000 @ 0000000005000000 (usable) memory: 0000000020000000 @ 0000000050000000 (usable) ------ mem_section[0][0]:0x8000000008184003 mem_section[1][0]:0x0 mem_section[2][0]:0x0 mem_section[3][0]:0x0 mem_section[4][0]:0x0 mem_section[5][0]:0x8000000007384003 mem_section[6][0]:0x8000000007384003 mem_section[7][0]:0x0 << Repeats rest of 1024 times >>
