On 29.07.20 11:31, Mike Rapoport wrote: > Hi Justin, > > On Wed, Jul 29, 2020 at 08:27:58AM +0000, Justin He wrote: >> Hi David >>>> >>>> Without this series, if qemu creates a 4G bytes nvdimm device, we can >>> only >>>> use 2G bytes for dax pmem(kmem) in the worst case. >>>> e.g. >>>> 240000000-33fdfffff : Persistent Memory >>>> We can only use the memblock between [240000000, 2ffffffff] due to the >>> hard >>>> limitation. It wastes too much memory space. >>>> >>>> Decreasing the SECTION_SIZE_BITS on arm64 might be an alternative, but >>> there >>>> are too many concerns from other constraints, e.g. PAGE_SIZE, hugetlb, >>>> SPARSEMEM_VMEMMAP, page bits in struct page ... >>>> >>>> Beside decreasing the SECTION_SIZE_BITS, we can also relax the kmem >>> alignment >>>> with memory_block_size_bytes(). >>>> >>>> Tested on arm64 guest and x86 guest, qemu creates a 4G pmem device. dax >>> pmem >>>> can be used as ram with smaller gap. Also the kmem hotplug add/remove >>> are both >>>> tested on arm64/x86 guest. >>>> >>> >>> Hi, >>> >>> I am not convinced this use case is worth such hacks (that’s what it is) >>> for now. On real machines pmem is big - your example (losing 50% is >>> extreme). >>> >>> I would much rather want to see the section size on arm64 reduced. I >>> remember there were patches and that at least with a base page size of 4k >>> it can be reduced drastically (64k base pages are more problematic due to >>> the ridiculous THP size of 512M). But could be a section size of 512 is >>> possible on all configs right now. >> >> Yes, I once investigated how to reduce section size on arm64 thoughtfully: >> There are many constraints for reducing SECTION_SIZE_BITS >> 1. Given page->flags bits is limited, SECTION_SIZE_BITS can't be reduced too >> much. >> 2. Once CONFIG_SPARSEMEM_VMEMMAP is enabled, section id will not be counted >> into page->flags. >> 3. MAX_ORDER depends on SECTION_SIZE_BITS >> - 3.1 mmzone.h >> #if (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS >> #error Allocator MAX_ORDER exceeds SECTION_SIZE >> #endif >> - 3.2 hugepage_init() >> MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER >= MAX_ORDER); >> >> Hence when ARM64_4K_PAGES && CONFIG_SPARSEMEM_VMEMMAP are enabled, >> SECTION_SIZE_BITS can be reduced to 27. >> But when ARM64_64K_PAGES, given 3.2, MAX_ORDER > 29-16 = 13. >> Given 3.1 SECTION_SIZE_BITS >= MAX_ORDER+15 > 28. So SECTION_SIZE_BITS can not >> be reduced to 27. >> >> In one word, if we considered to reduce SECTION_SIZE_BITS on arm64, the Kconfig >> might be very complicated,e.g. we still need to consider the case for >> ARM64_16K_PAGES. > > It is not necessary to pollute Kconfig with that. > arch/arm64/include/asm/sparesemem.h can have something like > > #ifdef CONFIG_ARM64_64K_PAGES > #define SPARSE_SECTION_SIZE 29 > #elif defined(CONFIG_ARM16K_PAGES) > #define SPARSE_SECTION_SIZE 28 > #elif defined(CONFIG_ARM4K_PAGES) > #define SPARSE_SECTION_SIZE 27 > #else > #error > #endif ack > > There is still large gap with ARM64_64K_PAGES, though. > > As for SPARSEMEM without VMEMMAP, are there actual benefits to use it? I was asking myself the same question a while ago and didn't really find a compelling one. I think it's always enabled as default (SPARSEMEM_VMEMMAP_ENABLE) and would require config tweaks to even disable it. -- Thanks, David / dhildenb
