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

[Justin He <Justin.He@arm.com>]

Hi David

> -----Original Message-----
> From: David Hildenbrand <david@redhat.com>
> Sent: Wednesday, July 29, 2020 2:37 PM
> To: Justin He <Justin.He@arm.com>
> Cc: Dan Williams <dan.j.williams@intel.com>; Vishal Verma
> <vishal.l.verma@intel.com>; Mike Rapoport <rppt@linux.ibm.com>; David
> Hildenbrand <david@redhat.com>; Catalin Marinas <Catalin.Marinas@arm.com>;
> Will Deacon <will@kernel.org>; Greg Kroah-Hartman
> <gregkh@linuxfoundation.org>; Rafael J. Wysocki <rafael@kernel.org>; Dave
> Jiang <dave.jiang@intel.com>; Andrew Morton <akpm@linux-foundation.org>;
> Steve Capper <Steve.Capper@arm.com>; Mark Rutland <Mark.Rutland@arm.com>;
> Logan Gunthorpe <logang@deltatee.com>; Anshuman Khandual
> <Anshuman.Khandual@arm.com>; Hsin-Yi Wang <hsinyi@chromium.org>; Jason
> Gunthorpe <jgg@ziepe.ca>; Dave Hansen <dave.hansen@linux.intel.com>; Kees
> Cook <keescook@chromium.org>; linux-arm-kernel@lists.infradead.org; linux-
> kernel@vger.kernel.org; linux-nvdimm@lists.01.org; linux-mm@kvack.org; Wei
> Yang <richardw.yang@linux.intel.com>; Pankaj Gupta
> <pankaj.gupta.linux@gmail.com>; Ira Weiny <ira.weiny@intel.com>; Kaly Xin
> <Kaly.Xin@arm.com>
> Subject: Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem
> alignment
> 
> 
> 
> > Am 29.07.2020 um 05:35 schrieb Jia He <justin.he@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.

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.

> 
> 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
> >

_______________________________________________
Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org
To unsubscribe send an email to linux-nvdimm-leave@lists.01.org