On Wed, Jul 29, 2020 at 11:35:20AM +0200, David Hildenbrand wrote: > 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. Memory overhead for VMEMMAP is larger, especially for arm64 that knows how to free empty parts of the memory map with "classic" SPARSEMEM. > I think it's always enabled as default (SPARSEMEM_VMEMMAP_ENABLE) and > would require config tweaks to even disable it. Nope, it's right there in menuconfig, "Memory Management options" -> "Sparse Memory virtual memmap" > -- > Thanks, > > David / dhildenb > -- Sincerely yours, Mike.
