2 files changed, 19 insertions, 10 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90bb644..24d01648edeb 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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
+                        fall back to reserve region above 4G when '@offset'
+                        hasn't been specified.
+                        See Documentation/kdump/kdump.txt for further details.
 
         crashkernel=range1:size1[,range2:size2,...][@offset]
                         [KNL] Same as above, but depends on the memory
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index daf7c5650c18..c15f362a2516 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -541,21 +541,27 @@ static void __init reserve_crashkernel(void)
         }
 
         /* 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.
+                 * crashkernel=x,high reserves memory over 4G, also allocates
+                 * 256M extra low memory for DMA buffers and swiotlb.
+                 * But the extra memory is not required for all machines.
+                 * So try low memory first and fall back 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;