On 29.07.20 15:00, Mike Rapoport wrote: > 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. You mean the hole punching within section memmap? (which is why their pfn_valid() implementation is special) (I do wonder why that shouldn't work with VMEMMAP, or is it simply not implemented?) > >> 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" Ah, good to know. -- Thanks, David / dhildenb
