arm64/sparsemem: reduce SECTION_SIZE_BITS

memory_block_size_bytes() determines the memory hotplug granularity i.e the
amount of memory which can be hot added or hot removed from the kernel. The
generic value here being MIN_MEMORY_BLOCK_SIZE (1UL << SECTION_SIZE_BITS)
for memory_block_size_bytes() on platforms like arm64 that does not override.

Current SECTION_SIZE_BITS is 30 i.e 1GB which is large and a reduction here
increases memory hotplug granularity, thus improving its agility. A reduced
section size also reduces memory wastage in vmemmmap mapping for sections
with large memory holes. So we try to set the least section size as possible.

A section size bits selection must follow:
(MAX_ORDER - 1 + PAGE_SHIFT) <= SECTION_SIZE_BITS

CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 and so just following it
would help achieve the smallest section size.

SECTION_SIZE_BITS = (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)

SECTION_SIZE_BITS = 22 (11 - 1 + 12) i.e 4MB   for 4K pages
SECTION_SIZE_BITS = 24 (11 - 1 + 14) i.e 16MB  for 16K pages without THP
SECTION_SIZE_BITS = 25 (12 - 1 + 14) i.e 32MB  for 16K pages with THP
SECTION_SIZE_BITS = 26 (11 - 1 + 16) i.e 64MB  for 64K pages without THP
SECTION_SIZE_BITS = 29 (14 - 1 + 16) i.e 512MB for 64K pages with THP

But there are other problems in reducing SECTION_SIZE_BIT. Reducing it by too
much would over populate /sys/devices/system/memory/ and also consume too many
page->flags bits in the !vmemmap case. Also section size needs to be multiple
of 128MB to have PMD based vmemmap mapping with CONFIG_ARM64_4K_PAGES.

Given these constraints, lets just reduce the section size to 128MB for 4K
and 16K base page size configs, and to 512MB for 64K base page size config.

Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
Suggested-by: Anshuman Khandual <anshuman.khandual@arm.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/43843c5e092bfe3ec4c41e3c8c78a7ee35b69bb0.1611206601.git.sudaraja@codeaurora.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/sparsemem.h

@@ -7,7 +7,26 @@
 
 #ifdef CONFIG_SPARSEMEM
 #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
-#define SECTION_SIZE_BITS	30
-#endif
+
+/*
+ * Section size must be at least 512MB for 64K base
+ * page size config. Otherwise it will be less than
+ * (MAX_ORDER - 1) and the build process will fail.
+ */
+#ifdef CONFIG_ARM64_64K_PAGES
+#define SECTION_SIZE_BITS 29
+
+#else
+
+/*
+ * Section size must be at least 128MB for 4K base
+ * page size config. Otherwise PMD based huge page
+ * entries could not be created for vmemmap mappings.
+ * 16K follows 4K for simplicity.
+ */
+#define SECTION_SIZE_BITS 27
+#endif /* CONFIG_ARM64_64K_PAGES */
+
+#endif /* CONFIG_SPARSEMEM*/
 
 #endif