> Am 29.07.2020 um 05:35 schrieb Jia He <justin.he@xxxxxxx>: > > When enabling dax pmem as RAM device on arm64, I noticed that kmem_start > addr in dev_dax_kmem_probe() should be aligned w/ SECTION_SIZE_BITS(30),i.e. > 1G memblock size. Even Dan Williams' sub-section patch series [1] had been > upstream merged, it was not helpful due to hard limitation of kmem_start: > $ndctl create-namespace -e namespace0.0 --mode=devdax --map=dev -s 2g -f -a 2M > $echo dax0.0 > /sys/bus/dax/drivers/device_dax/unbind > $echo dax0.0 > /sys/bus/dax/drivers/kmem/new_id > $cat /proc/iomem > ... > 23c000000-23fffffff : System RAM > 23dd40000-23fecffff : reserved > 23fed0000-23fffffff : reserved > 240000000-33fdfffff : Persistent Memory > 240000000-2403fffff : namespace0.0 > 280000000-2bfffffff : dax0.0 <- aligned with 1G boundary > 280000000-2bfffffff : System RAM > Hence there is a big gap between 0x2403fffff and 0x280000000 due to the 1G > alignment. > > 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. 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 - 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 >
