[PATCH 2/2] X86/kdump: fall back to reserve high crashkernel memory

[Dave Young <dyoung@redhat.com>]

crashkernel=xM tries to reserve crashkernel memory under 4G, which
is enough for usual cases.  But this could fail sometimes, for example
one tries to reserve a big chunk like 2G, it is possible to fail.

So let the crashkernel=xM just fall back to use high memory in case it
fails to find a suitable low range.  Do not set the ,high as default
because it allocs extra low memory for DMA buffers and swiotlb, this is
not always necessary for all machines. Typically like crashkernel=128M
usually work with low reservation under 4G, so still keep <4G as default.

Signed-off-by: Dave Young <dyoung@redhat.com>
---
 Documentation/admin-guide/kernel-parameters.txt |    7 +++++--
 arch/x86/kernel/setup.c                         |   22 ++++++++++++++--------
 2 files changed, 19 insertions(+), 10 deletions(-)

--- linux-x86.orig/arch/x86/kernel/setup.c
+++ linux-x86/arch/x86/kernel/setup.c
@@ -541,21 +541,27 @@ static void __init reserve_crashkernel(v
 	}
 
 	/* 0 means: find the address automatically */
-	if (crash_base <= 0) {
+	if (!crash_base) {
 		/*
 		 * Set CRASH_ADDR_LOW_MAX upper bound for crash memory,
-		 * as old kexec-tools loads bzImage below that, unless
-		 * "crashkernel=size[KMG],high" is specified.
+		 * as crashkernel=x,high allocs memory over 4G, also allocs
+		 * 256M extra low memory for DMA buffers and swiotlb.
+		 * but the extra memory is not required for all machines.
+		 * So prefer low memory first, and fallback to high memory
+		 * unless "crashkernel=size[KMG],high" is specified.
 		 */
-		crash_base = memblock_find_in_range(CRASH_ALIGN,
-						    high ? CRASH_ADDR_HIGH_MAX
-							 : CRASH_ADDR_LOW_MAX,
-						    crash_size, CRASH_ALIGN);
+		if (!high)
+			crash_base = memblock_find_in_range(CRASH_ALIGN,
+						CRASH_ADDR_LOW_MAX,
+						crash_size, CRASH_ALIGN);
+		if (!crash_base)
+			crash_base = memblock_find_in_range(CRASH_ALIGN,
+						CRASH_ADDR_HIGH_MAX,
+						crash_size, CRASH_ALIGN);
 		if (!crash_base) {
 			pr_info("crashkernel reservation failed - No suitable area found.\n");
 			return;
 		}
-
 	} else {
 		unsigned long long start;
 
--- linux-x86.orig/Documentation/admin-guide/kernel-parameters.txt
+++ linux-x86/Documentation/admin-guide/kernel-parameters.txt
@@ -704,8 +704,11 @@
 			upon panic. This parameter reserves the physical
 			memory region [offset, offset + size] for that kernel
 			image. If '@offset' is omitted, then a suitable offset
-			is selected automatically. Check
-			Documentation/kdump/kdump.txt for further details.
+			is selected automatically.
+			[KNL, x86_64] select a region under 4G first, and
+			fallback to reserve region above 4G in case without
+			'@offset'.
+			See Documentation/kdump/kdump.txt for further details.
 
 	crashkernel=range1:size1[,range2:size2,...][@offset]
 			[KNL] Same as above, but depends on the memory