@@ -485,20 +485,20 @@ static void __init reserve_crashkernel(void)
 	if (flag == CRASHKERNEL_MEM_NONE)
 		return;
 
-	/* 0 means: find the address automatically */
 	if (!crash_base) {
 		/*
-		 * Set CRASH_ADDR_LOW_MAX upper bound for crash memory,
-		 * 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.
+		 * For the case of crashkernel=X[@offset] and offset is omitted,
+		 * try the low memory first.
 		 */
 		if (!(flag & CRASHKERNEL_MEM_HIGH))
 			crash_base = memblock_phys_alloc_range(crash_size,
 						CRASH_ALIGN, CRASH_ALIGN,
 						CRASH_ADDR_LOW_MAX);
+
+		/*
+		 * If low memory allocation failed above, or for the case of
+		 * crashkernel=X,high, try the high memory.
+		 */
 		if (!crash_base)
 			crash_base = memblock_phys_alloc_range(crash_size,
 						CRASH_ALIGN, CRASH_ALIGN,

@@ -510,6 +510,10 @@ static void __init reserve_crashkernel(void)
 	} else {
 		unsigned long long start;
 
+		/*
+		 * The case of crashkernel=X@offset and offset is specified.
+		 * Only user-specified space can be reserved.
+		 */
 		start = memblock_phys_alloc_range(crash_size, SZ_1M, crash_base,
 						  crash_base + crash_size);
 		if (start != crash_base) {

@@ -520,6 +524,10 @@ static void __init reserve_crashkernel(void)
 
 #ifdef CONFIG_X86_64
 	if (crash_base >= (1ULL << 32)) {
+		/*
+		 * Ensure that at least 256M extra low memory is allocated for
+		 * DMA buffers and swiotlb, if low memory size is not specified.
+		 */
 		if (!(flag & CRASHKERNEL_MEM_LOW)) {
 			/*
 			 * two parts from kernel/dma/swiotlb.c: