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

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

> -----Original Message-----
> From: David Hildenbrand <david@redhat.com>
> Sent: Wednesday, July 29, 2020 5:35 PM
> To: Mike Rapoport <rppt@linux.ibm.com>; Justin He <Justin.He@arm.com>
> Cc: Dan Williams <dan.j.williams@intel.com>; Vishal Verma
> <vishal.l.verma@intel.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
> 
> On 29.07.20 11:31, Mike Rapoport wrote:
> > 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
> 
> ack
Thanks, David and Mike. Will discuss it further more with arm internally about
the thoughtful section_size change

--
Cheers,
Justin (Jia He)

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