On 2020/10/6 1:12, Catalin Marinas wrote:

On Mon, Sep 07, 2020 at 09:47:43PM +0800, Chen Zhou wrote:

quoted

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 3f735cb37ace..d11d597a470d 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c

@@ -378,6 +378,15 @@ int __init reserve_crashkernel_low(void)
 }
 
 #if defined(CONFIG_X86) || defined(CONFIG_ARM64)
+
+/*
+ * Add a threshold for required memory size of crashkernel. If required memory
+ * size is greater than threshold, just go for high allocation directly. The
+ * value of threshold is set as half of the total low memory.
+ */
+#define REQUIRED_MEMORY_THRESHOLD	(memblock_mem_size(CRASH_ADDR_LOW_MAX >> \
+			PAGE_SHIFT) >> 1)
+
 #ifdef CONFIG_KEXEC_CORE
 /*
  * reserve_crashkernel() - reserves memory for crash kernel

@@ -422,7 +431,7 @@ void __init reserve_crashkernel(void)
 		 * So try low memory first and fall back to high memory
 		 * unless "crashkernel=size[KMG],high" is specified.
 		 */
-		if (!high)
+		if (!high && crash_size <= REQUIRED_MEMORY_THRESHOLD)
 			crash_base = memblock_find_in_range(CRASH_ALIGN,
 						CRASH_ADDR_LOW_MAX,
 						crash_size, CRASH_ALIGN);

Since any change now is affecting the x86 semantics slightly, I'd
suggest you drop this patch. We can add it later if needed, once the
core changes are in.

Ok, i will drop this patch in next version.

Thanks,
Chen Zhou

Thinking about this, if one requires a crashkernel reservation that
allocates all of the ZONE_DMA, it would probably be noticed and explicit
,high/,low options can be used.

Note that we are also trying to make ZONE_DMA full 32-bit on non-RPi4
hardware.