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

[Mike Rapoport <rppt@linux.ibm.com>]

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.