On 11.01.21 05:17, Anshuman Khandual wrote: > > > On 1/8/21 9:00 PM, David Hildenbrand wrote: >>> 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. > > Okay sure, will send a RFC to begin with. > Note that there is https://lkml.kernel.org/r/15cf9a2359197fee0168f820c5c904650d07939e.1610146597.git.sudaraja@xxxxxxxxxxxxxx (Sudarshan missed to cc linux-mm) -- Thanks, David / dhildenb
