Re: [PATCH 1/1] arm64: reduce section size for sparsemem

Reducing the section size helps reduce wastage of reserved memory
for huge memory holes in sparsemem model. But having a much smaller
section size bits could break PMD mappings for vmemmap and wouldn't
accomodate the highest order page for certain page size granule configs.
It is determined that SECTION_SIZE_BITS of 27 (128MB) could be ideal
default value for 4K_PAGES that gives least section size without breaking
PMD based vmemmap mappings. For simplicity, 16K_PAGES could follow the
same as 4K_PAGES. And the least SECTION_SIZE_BITS for 64K_PAGES is 29
that could accomodate MAX_ORDER.

Signed-off-by: Sudarshan Rajagopalan <redacted>
Suggested-by: David Hildenbrand <redacted>
Cc: Will Deacon <will@kernel.org>
Cc: Anshuman Khandual <redacted>
Cc: Mike Rapoport <redacted>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
---
 arch/arm64/include/asm/sparsemem.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
index 1f43fcc79738..ff08ff6b677c 100644
--- a/arch/arm64/include/asm/sparsemem.h
+++ b/arch/arm64/include/asm/sparsemem.h

@@ -7,7 +7,13 @@
 
 #ifdef CONFIG_SPARSEMEM
 #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
-#define SECTION_SIZE_BITS	30
-#endif
+
+#if defined(CONFIG_ARM64_4K_PAGES) || defined(CONFIG_ARM64_16K_PAGES)
+#define SECTION_SIZE_BITS 27
+#else
+#define SECTION_SIZE_BITS 29
+#endif /* CONFIG_ARM64_4K_PAGES || CONFIG_ARM64_16K_PAGES */
+
+#endif /* CONFIG_SPARSEMEM*/