Now crashkernel=X will fail if there's not enough memory at low region (below 896M) when trying to reserve large memory size. One can use crashkernel=xM,high to reserve it at high region (>4G) but it is more convinient to improve crashkernel=X to: - First try to reserve X below 896M (for being compatible with old kexec-tools). - If fails, try to reserve X below 4G (swiotlb need to stay below 4G). - If fails, try to reserve X from MAXMEM top down. It's more transparent and user-friendly. If crashkernel is large and the reserved is beyond 896M, old kexec-tools is not compatible with new kernel because old kexec-tools can not load kernel at high memory region, there was an old discussion below: https://lkml.org/lkml/2013/10/15/601 But actually the behavior is consistent during my test. Suppose old kernel fail to reserve memory at low areas, kdump does not work because no meory reserved. With this patch, suppose new kernel successfully reserved memory at high areas, old kexec-tools still fail to load kdump kernel (tested 2.0.2), so it is acceptable, no need to worry about the compatibility. Here is the test result (kexec-tools 2.0.2, no high memory load support): Crashkernel over 4G: # cat /proc/iomem|grep Crash be000000-cdffffff : Crash kernel 213000000-21effffff : Crash kernel # ./kexec -p /boot/vmlinuz-`uname -r` Memory for crashkernel is not reserved Please reserve memory by passing "crashkernel=X at Y" parameter to the kernel Then try loading kdump kernel crashkernel: 896M-4G: # cat /proc/iomem|grep Crash 96000000-cdefffff : Crash kernel # ./kexec -p /boot/vmlinuz-4.14.0-rc4+ ELF core (kcore) parse failed Cannot load /boot/vmlinuz-4.14.0-rc4+ Signed-off-by: Dave Young <dyoung at redhat.com> --- arch/x86/kernel/setup.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) --- linux-x86.orig/arch/x86/kernel/setup.c +++ linux-x86/arch/x86/kernel/setup.c @@ -568,6 +568,22 @@ static void __init reserve_crashkernel(v high ? CRASH_ADDR_HIGH_MAX : CRASH_ADDR_LOW_MAX, crash_size, CRASH_ALIGN); +#ifdef CONFIG_X86_64 + /* + * crashkernel=X reserve below 896M fails? Try below 4G + */ + if (!high && !crash_base) + crash_base = memblock_find_in_range(CRASH_ALIGN, + (1ULL << 32), + crash_size, CRASH_ALIGN); + /* + * crashkernel=X reserve below 4G fails? Try MAXMEM + */ + if (!high && !crash_base) + crash_base = memblock_find_in_range(CRASH_ALIGN, + CRASH_ADDR_HIGH_MAX, + crash_size, CRASH_ALIGN); +#endif if (!crash_base) { pr_info("crashkernel reservation failed - No suitable area found.\n"); return;
