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@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@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;