> To summarize, the section size bits for each base page size config > should always > > a. Avoid (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS Pageblocks must also always fall completely into a section. > > b. Provide minimum possible section size for a given base page config to > have increased agility during memory hotplug operations and reduced > vmemmap wastage for sections with holes. OTOH, making the section size too small (e.g., 16MB) creates way to many memory block devices in /sys/devices/system/memory/, and might consume too many page->flags bits in the !vmemmap case. For bigger setups, we might, similar to x86-64 (e.g., >= 64 GiB), determine the memory_block_size_bytes() at runtime (spanning multiple sections then), once it becomes relevant. > > c. Allow 4K base page configs to have PMD based vmemmap mappings Agreed. > > Because CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 platform, > the following would always avoid the condition (a) > > SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) > > - 22 (11 - 1 + 12) for 4K pages > - 24 (11 - 1 + 14) for 16K pages without THP > - 25 (12 - 1 + 14) for 16K pages with THP > - 26 (11 - 1 + 16) for 64K pages without THP > - 29 (14 - 1 + 16) for 64K pages with THP > > Apart from overriding 4K base page size config to have 27 as section size > bits, should not all other values be okay here ? But then wondering what > benefit 128MB (27 bits) section size for 16K config would have ? OR the > objective here is to match 16K page size config with default x86-64. We don't want to have sections that are too small. We don't want to have sections that are too big :) Not sure if we really want to allow setting e.g., a section size of 4 MB. That's just going to hurt. IMHO, something in the range of 64..256 MB is quite a good choice, where possible. > >> >> (If we worry about the number of section bits in page->flags, we could >> glue it to vmemmap support where that does not matter) > > Could you please elaborate ? Could smaller section size bits numbers like > 22 or 24 create problems in page->flags without changing other parameters > like NR_CPUS or NODES_SHIFT ? A quick test with 64K base page without THP Yes, in the !vmemmap case, we have to store the section_nr in there. IIRC, it's less of an issue with section sizes like 128 MB. > i.e 26 bits in section size, fails to boot. 26 bits would mean 64 MB, no? Not sure if that's possible even without THP (MAX_ORDER - 1, pageblock_order ...) on 64k pages. I'd assume 512 MB is the lowest we can go. I'd assume this would crash :) > > As you have suggested, probably constant defaults (128MB for 4K/16K, 512MB > for 64K) might be better than depending on the CONFIG_FORCE_MAX_ZONEORDER, > at least for now. That's also what I would prefer, keeping it simple. -- Thanks, David / dhildenb
