On Fri, Dec 04, 2020 at 11:13:47AM +0000, Will Deacon wrote:

On Fri, Dec 04, 2020 at 09:44:43AM +0800, Wei Li wrote:

quoted

For the memory hole, sparse memory model that define SPARSEMEM_VMEMMAP
do not free the reserved memory for the page map, decrease the section
size can reduce the waste of reserved memory.

Signed-off-by: Wei Li <redacted>
Signed-off-by: Baopeng Feng <redacted>
Signed-off-by: Xia Qing <redacted>
---
 arch/arm64/include/asm/sparsemem.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
index 1f43fcc79738..8963bd3def28 100644
--- a/arch/arm64/include/asm/sparsemem.h
+++ b/arch/arm64/include/asm/sparsemem.h

@@ -7,7 +7,7 @@

 #ifdef CONFIG_SPARSEMEM
 #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
-#define SECTION_SIZE_BITS	30
+#define SECTION_SIZE_BITS	27

We chose '30' to avoid running out of bits in the page flags. What changed?

I think that for 64-bit there are still plenty of free bits. I didn't
check now, but when I played with SPARSEMEM on m68k there were 8 bits
for section out of 32.

With this patch, I can trigger:

./include/linux/mmzone.h:1170:2: error: Allocator MAX_ORDER exceeds SECTION_SIZE
#error Allocator MAX_ORDER exceeds SECTION_SIZE

if I bump up NR_CPUS and NODES_SHIFT.

I don't think it's related to NR_CPUS and NODES_SHIFT. 
This seems rather 64K pages that cause this.

Not that is shouldn't be addressed.

Will

-- 
Sincerely yours,
Mike.

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