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 There is still large gap with ARM64_64K_PAGES, though. As for SPARSEMEM without VMEMMAP, are there actual benefits to use it? > > > > In the long term we might want to rework the memory block device model > > (eventually supporting old/new as discussed with Michal some time ago > > using a kernel parameter), dropping the fixed sizes > > Has this been posted to Linux mm maillist? Sorry, searched and didn't find it. > > > -- > Cheers, > Justin (Jia He) > > > > > - allowing sizes / addresses aligned with subsection size > > - drastically reducing the number of devices for boot memory to only a > > hand full (e.g., one per resource / DIMM we can actually unplug again. > > > > Long story short, I don’t like this hack. > > > > > > > This patch series (mainly patch6/6) is based on the fixing patch, ~v5.8- > > rc5 [2]. > > > > > > [1] https://lkml.org/lkml/2019/6/19/67 > > > [2] https://lkml.org/lkml/2020/7/8/1546 > > > Jia He (6): > > > mm/memory_hotplug: remove redundant memory block size alignment check > > > resource: export find_next_iomem_res() helper > > > mm/memory_hotplug: allow pmem kmem not to align with memory_block_size > > > mm/page_alloc: adjust the start,end in dax pmem kmem case > > > device-dax: relax the memblock size alignment for kmem_start > > > arm64: fall back to vmemmap_populate_basepages if not aligned with > > > PMD_SIZE > > > > > > arch/arm64/mm/mmu.c | 4 ++++ > > > drivers/base/memory.c | 24 ++++++++++++++++-------- > > > drivers/dax/kmem.c | 22 +++++++++++++--------- > > > include/linux/ioport.h | 3 +++ > > > kernel/resource.c | 3 ++- > > > mm/memory_hotplug.c | 39 ++++++++++++++++++++++++++++++++++++++- > > > mm/page_alloc.c | 14 ++++++++++++++ > > > 7 files changed, 90 insertions(+), 19 deletions(-) > > > > > > -- > > > 2.17.1 > > > > -- Sincerely yours, Mike.
