arm64: kdump: Do not allocate crash low memory if not needed

When "crashkernel=X,high" is specified, the specified "crashkernel=Y,low"
memory is not required in the following corner cases:
1. If both CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32 are disabled, it means
   that the devices can access any memory.
2. If the system memory is small, the crash high memory may be allocated
   from the DMA zones. If that happens, there's no need to allocate
   another crash low memory because there's already one.

Add condition '(crash_base >= CRASH_ADDR_LOW_MAX)' to determine whether
the 'high' memory is allocated above DMA zones. Note: when both
CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32 are disabled, the entire physical
memory is DMA accessible, CRASH_ADDR_LOW_MAX equals 'PHYS_MASK + 1'.
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/20220511032033.426-1-thunder.leizhen@huawei.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -823,7 +823,7 @@
 			low memory is needed to make sure DMA buffers for 32-bit
 			devices won't run out. Kernel would try to allocate
 			at least 256M below 4G automatically.
-			This one let user to specify own low range under 4G
+			This one lets the user specify own low range under 4G
 			for second kernel instead.
 			0: to disable low allocation.
 			It will be ignored when crashkernel=X,high is not used
@@ -832,7 +832,8 @@
 			[KNL, ARM64] range in low memory.
 			This one lets the user specify a low range in the
 			DMA zone for the crash dump kernel.
-			It will be ignored when crashkernel=X,high is not used.
+			It will be ignored when crashkernel=X,high is not used
+			or memory reserved is located in the DMA zones.
 
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests

arch/arm64/mm/init.c

@@ -170,7 +170,8 @@ static void __init reserve_crashkernel(void)
 		return;
 	}
 
-	if (crash_low_size && reserve_crashkernel_low(crash_low_size)) {
+	if ((crash_base >= CRASH_ADDR_LOW_MAX) &&
+	     crash_low_size && reserve_crashkernel_low(crash_low_size)) {
 		memblock_phys_free(crash_base, crash_size);
 		return;
 	}