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From:   David Hildenbrand <david@redhat.com>
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Subject: Re: [RFC PATCH 0/6] decrease unnecessary gap due to pmem kmem alignment
Date:   Wed, 29 Jul 2020 08:36:36 +0200
Message-Id: <D1981D47-61F1-42E9-A426-6FEF0EC310C8@redhat.com>
References: <20200729033424.2629-1-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>
In-Reply-To: <20200729033424.2629-1-justin.he@arm.com>
To:     Jia He <justin.he@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> Am 29.07.2020 um 05:35 schrieb Jia He <justin.he@arm.com>:
>=20
> =EF=BB=BFWhen enabling dax pmem as RAM device on arm64, I noticed that kme=
m_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=3Ddevdax --map=3Ddev -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.
>=20
> 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=20
> We can only use the memblock between [240000000, 2ffffffff] due to the har=
d
> limitation. It wastes too much memory space.
>=20
> Decreasing the SECTION_SIZE_BITS on arm64 might be an alternative, but the=
re
> are too many concerns from other constraints, e.g. PAGE_SIZE, hugetlb,
> SPARSEMEM_VMEMMAP, page bits in struct page ...
>=20
> Beside decreasing the SECTION_SIZE_BITS, we can also relax the kmem alignm=
ent
> with memory_block_size_bytes().
>=20
> Tested on arm64 guest and x86 guest, qemu creates a 4G pmem device. dax pm=
em
> can be used as ram with smaller gap. Also the kmem hotplug add/remove are b=
oth
> tested on arm64/x86 guest.
>=20

Hi,

I am not convinced this use case is worth such hacks (that=E2=80=99s what it=
 is) for now. On real machines pmem is big - your example (losing 50% is ext=
reme).

I would much rather want to see the section size on arm64 reduced. I remembe=
r 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 ridicul=
ous 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 (even=
tually supporting old/new as discussed with Michal some time ago using a ker=
nel 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 f=
ull (e.g., one per resource / DIMM we can actually unplug again.

Long story short, I don=E2=80=99t like this hack.


> This patch series (mainly patch6/6) is based on the fixing patch, ~v5.8-rc=
5 [2].
>=20
> [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
>=20
> 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(-)
>=20
> --=20
> 2.17.1
>=20