On 12/7/20 7:10 AM, Song Bao Hua (Barry Song) wrote:

quoted

-----Original Message-----
From: Mike Rapoport [mailto:rppt@linux.ibm.com]
Sent: Saturday, December 5, 2020 12:44 AM
To: Will Deacon <will@kernel.org>
Cc: liwei (CM) <redacted>; catalin.marinas@arm.com; fengbaopeng
[off-list ref]; nsaenzjulienne@suse.de; steve.capper@arm.com;
Song Bao Hua (Barry Song) [off-list ref];
linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; butao
[off-list ref]
Subject: Re: [PATCH] arm64: mm: decrease the section size to reduce the memory
reserved for the page map

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

quoted

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

quoted

quoted

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.

quoted

With this patch, I can trigger:

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

SECTION_SIZE

quoted

#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.

Right now, only 4K PAGES will define ARM64_SWAPPER_USES_SECTION_MAPS.
Other cases will use vmemmap_populate_basepages().
The original patch should be only addressing the issue in 4K pages:
https://lore.kernel.org/lkml/20200812010655.96339-1-liwei213@huawei.com/ (local)

would we do something like the below?
#ifdef CONFIG_ARM64_4K_PAGE
#define SECTION_SIZE_BITS	27
#else
#define SECTION_SIZE_BITS	30
#endif

This is bit arbitrary. Probably 27 can be further reduced for 4K page size.
Instead, we should make SECTION_SIZE_BITS explicitly depend upon MAX_ORDER.
IOW section size should be the same as the highest order page in the buddy.
CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64. A quick test shows
SECTION_SIZE_BITS would be 22 on 4K pages and 29 for 64K pages. As a fall
back SECTION_SIZE_BITS can still be 30 in case CONFIG_FORCE_MAX_ZONEORDER
is not defined.

--- 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      (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
 #endif
 
 #endif

A similar approach exists on ia64 platform as well.

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