[RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment

[David Hildenbrand]

> Am 29.07.2020 um 05:35 schrieb Jia He <justin.he at arm.com>:
> 
> 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
>