From: Mike Rapoport <rppt@linux.ibm.com>
To: David Hildenbrand <david@redhat.com>
Cc: Justin He <Justin.He@arm.com>,
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-arm-kernel@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
"linux-mm@kvack.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
Date: Wed, 29 Jul 2020 16:00:25 +0300 [thread overview]
Message-ID: <20200729130025.GD3672596@linux.ibm.com> (raw)
In-Reply-To: <e128f304-7d1d-c1eb-2def-fee7d105424f@redhat.com>
On Wed, Jul 29, 2020 at 11:35:20AM +0200, David Hildenbrand wrote:
> 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
>
> >
> > There is still large gap with ARM64_64K_PAGES, though.
> >
> > As for SPARSEMEM without VMEMMAP, are there actual benefits to use it?
>
> I was asking myself the same question a while ago and didn't really find
> a compelling one.
Memory overhead for VMEMMAP is larger, especially for arm64 that knows
how to free empty parts of the memory map with "classic" SPARSEMEM.
> I think it's always enabled as default (SPARSEMEM_VMEMMAP_ENABLE) and
> would require config tweaks to even disable it.
Nope, it's right there in menuconfig,
"Memory Management options" -> "Sparse Memory virtual memmap"
> --
> Thanks,
>
> David / dhildenb
>
--
Sincerely yours,
Mike.
next prev parent reply other threads:[~2020-07-29 13:01 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-29 3:34 [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment Jia He
2020-07-29 3:34 ` [RFC PATCH 1/6] mm/memory_hotplug: remove redundant memory block size alignment check Jia He
2020-07-29 3:34 ` [RFC PATCH 2/6] resource: export find_next_iomem_res() helper Jia He
2020-07-29 3:34 ` [RFC PATCH 3/6] mm/memory_hotplug: allow pmem kmem not to align with memory_block_size Jia He
2020-07-29 3:34 ` [RFC PATCH 4/6] mm/page_alloc: adjust the start,end in dax pmem kmem case Jia He
2020-07-29 3:34 ` [RFC PATCH 5/6] device-dax: relax the memblock size alignment for kmem_start Jia He
2020-07-29 3:34 ` [RFC PATCH 6/6] arm64: fall back to vmemmap_populate_basepages if not aligned with PMD_SIZE Jia He
2020-07-29 6:36 ` [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment David Hildenbrand
2020-07-29 8:27 ` Justin He
2020-07-29 8:44 ` David Hildenbrand
2020-07-29 9:31 ` Mike Rapoport
2020-07-29 9:35 ` David Hildenbrand
2020-07-29 13:00 ` Mike Rapoport [this message]
2020-07-29 13:03 ` David Hildenbrand
2020-07-29 14:12 ` Mike Rapoport
2020-07-30 2:17 ` Justin He
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200729130025.GD3672596@linux.ibm.com \
--to=rppt@linux.ibm.com \
--cc=Anshuman.Khandual@arm.com \
--cc=Catalin.Marinas@arm.com \
--cc=Justin.He@arm.com \
--cc=Kaly.Xin@arm.com \
--cc=Mark.Rutland@arm.com \
--cc=Steve.Capper@arm.com \
--cc=akpm@linux-foundation.org \
--cc=dan.j.williams@intel.com \
--cc=dave.hansen@linux.intel.com \
--cc=dave.jiang@intel.com \
--cc=david@redhat.com \
--cc=gregkh@linuxfoundation.org \
--cc=hsinyi@chromium.org \
--cc=ira.weiny@intel.com \
--cc=jgg@ziepe.ca \
--cc=keescook@chromium.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-nvdimm@lists.01.org \
--cc=logang@deltatee.com \
--cc=pankaj.gupta.linux@gmail.com \
--cc=rafael@kernel.org \
--cc=richardw.yang@linux.intel.com \
--cc=vishal.l.verma@intel.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).