On 21.01.21 15:16, Mike Rapoport wrote: > On Wed, Jan 20, 2021 at 09:29:13PM -0800, Sudarshan Rajagopalan wrote: >> 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@xxxxxxxxxxxxxx> >> Suggested-by: Anshuman Khandual <anshuman.khandual@xxxxxxx> >> Suggested-by: David Hildenbrand <david@xxxxxxxxxx> >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> >> Cc: Will Deacon <will@xxxxxxxxxx> >> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx> >> Cc: David Hildenbrand <david@xxxxxxxxxx> >> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> >> Cc: Mark Rutland <mark.rutland@xxxxxxx> >> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: Steven Price <steven.price@xxxxxxx> >> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > BTW, after reduction of the section size maybe arm64 should consider opting > out of freeing unused memory map. > > This will make David even more happy as this will allow dropping custom > pfn_valid() ;-) Mike knows my wildest dreams ;) -- Thanks, David / dhildenb
