Message-Id: <20190513210148.GA21574@rapoport-lnx>
Date:   Tue, 14 May 2019 00:01:48 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, David Hildenbrand <david@...hat.com>,
        Jane Chu <jane.chu@...cle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Robin Murphy <robin.murphy@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Paul Mackerras <paulus@...ba.org>,
        Toshi Kani <toshi.kani@....com>,
        Oscar Salvador <osalvador@...e.de>,
        Jeff Moyer <jmoyer@...hat.com>, Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>, stable@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-nvdimm@...ts.01.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 00/12] mm: Sub-section memory hotplug support

Hi Dan,

On Mon, May 06, 2019 at 04:39:26PM -0700, Dan Williams wrote:
> Changes since v7 [1]:

Sorry for jumping late, but presuming there will be v9, it'd be great if it
would also include include updates to
Documentation/admin-guide/mm/memory-hotplug.rst and
Documentation/vm/memory-model.rst
 
> - Make subsection helpers pfn based rather than physical-address based
>   (Oscar and Pavel)
> 
> - Make subsection bitmap definition scalable for different section and
>   sub-section sizes across architectures. As a result:
> 
>       unsigned long map_active
> 
>   ...is converted to:
> 
>       DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION)
> 
>   ...and the helpers are renamed with a 'subsection' prefix. (Pavel)
> 
> - New in this version is a touch of arch/powerpc/include/asm/sparsemem.h
>   in "[PATCH v8 01/12] mm/sparsemem: Introduce struct mem_section_usage"
>   to define ARCH_SUBSECTION_SHIFT.
> 
> - Drop "mm/sparsemem: Introduce common definitions for the size and mask
>   of a section" in favor of Robin's "mm/memremap: Rename and consolidate
>   SECTION_SIZE" (Pavel)
> 
> - Collect some more Reviewed-by tags. Patches that still lack review
>   tags: 1, 3, 9 - 12
> 
> [1]: https://lore.kernel.org/lkml/155677652226.2336373.8700273400832001094.stgit@dwillia2-desk3.amr.corp.intel.com/
> 
> ---
> [merge logistics]
> 
> Hi Andrew,
> 
> These are too late for v5.2, I'm posting this v8 during the merge window
> to maintain the review momentum. 
> 
> ---
> [cover letter]
> 
> The memory hotplug section is an arbitrary / convenient unit for memory
> hotplug. 'Section-size' units have bled into the user interface
> ('memblock' sysfs) and can not be changed without breaking existing
> userspace. The section-size constraint, while mostly benign for typical
> memory hotplug, has and continues to wreak havoc with 'device-memory'
> use cases, persistent memory (pmem) in particular. Recall that pmem uses
> devm_memremap_pages(), and subsequently arch_add_memory(), to allocate a
> 'struct page' memmap for pmem. However, it does not use the 'bottom
> half' of memory hotplug, i.e. never marks pmem pages online and never
> exposes the userspace memblock interface for pmem. This leaves an
> opening to redress the section-size constraint.
> 
> To date, the libnvdimm subsystem has attempted to inject padding to
> satisfy the internal constraints of arch_add_memory(). Beyond
> complicating the code, leading to bugs [2], wasting memory, and limiting
> configuration flexibility, the padding hack is broken when the platform
> changes this physical memory alignment of pmem from one boot to the
> next. Device failure (intermittent or permanent) and physical
> reconfiguration are events that can cause the platform firmware to
> change the physical placement of pmem on a subsequent boot, and device
> failure is an everyday event in a data-center.
> 
> It turns out that sections are only a hard requirement of the
> user-facing interface for memory hotplug and with a bit more
> infrastructure sub-section arch_add_memory() support can be added for
> kernel internal usages like devm_memremap_pages(). Here is an analysis
> of the current design assumptions in the current code and how they are
> addressed in the new implementation:
> 
> Current design assumptions:
> 
> - Sections that describe boot memory (early sections) are never
>   unplugged / removed.
> 
> - pfn_valid(), in the CONFIG_SPARSEMEM_VMEMMAP=y, case devolves to a
>   valid_section() check
> 
> - __add_pages() and helper routines assume all operations occur in
>   PAGES_PER_SECTION units.
> 
> - The memblock sysfs interface only comprehends full sections
> 
> New design assumptions:
> 
> - Sections are instrumented with a sub-section bitmask to track (on x86)
>   individual 2MB sub-divisions of a 128MB section.
> 
> - Partially populated early sections can be extended with additional
>   sub-sections, and those sub-sections can be removed with
>   arch_remove_memory(). With this in place we no longer lose usable memory
>   capacity to padding.
> 
> - pfn_valid() is updated to look deeper than valid_section() to also check the
>   active-sub-section mask. This indication is in the same cacheline as
>   the valid_section() so the performance impact is expected to be
>   negligible. So far the lkp robot has not reported any regressions.
> 
> - Outside of the core vmemmap population routines which are replaced,
>   other helper routines like shrink_{zone,pgdat}_span() are updated to
>   handle the smaller granularity. Core memory hotplug routines that deal
>   with online memory are not touched.
> 
> - The existing memblock sysfs user api guarantees / assumptions are
>   not touched since this capability is limited to !online
>   !memblock-sysfs-accessible sections.
> 
> Meanwhile the issue reports continue to roll in from users that do not
> understand when and how the 128MB constraint will bite them. The current
> implementation relied on being able to support at least one misaligned
> namespace, but that immediately falls over on any moderately complex
> namespace creation attempt. Beyond the initial problem of 'System RAM'
> colliding with pmem, and the unsolvable problem of physical alignment
> changes, Linux is now being exposed to platforms that collide pmem
> ranges with other pmem ranges by default [3]. In short,
> devm_memremap_pages() has pushed the venerable section-size constraint
> past the breaking point, and the simplicity of section-aligned
> arch_add_memory() is no longer tenable.
> 
> These patches are exposed to the kbuild robot on my libnvdimm-pending
> branch [4], and a preview of the unit test for this functionality is
> available on the 'subsection-pending' branch of ndctl [5].
> 
> [2]: https://lore.kernel.org/r/155000671719.348031.2347363160141119237.stgit@dwillia2-desk3.amr.corp.intel.com
> [3]: https://github.com/pmem/ndctl/issues/76
> [4]: https://git.kernel.org/pub/scm/linux/kernel/git/djbw/nvdimm.git/log/?h=libnvdimm-pending
> [5]: https://github.com/pmem/ndctl/commit/7c59b4867e1c
> 
> ---
> 
> Dan Williams (11):
>       mm/sparsemem: Introduce struct mem_section_usage
>       mm/sparsemem: Add helpers track active portions of a section at boot
>       mm/hotplug: Prepare shrink_{zone,pgdat}_span for sub-section removal
>       mm/sparsemem: Convert kmalloc_section_memmap() to populate_section_memmap()
>       mm/hotplug: Kill is_dev_zone() usage in __remove_pages()
>       mm: Kill is_dev_zone() helper
>       mm/sparsemem: Prepare for sub-section ranges
>       mm/sparsemem: Support sub-section hotplug
>       mm/devm_memremap_pages: Enable sub-section remap
>       libnvdimm/pfn: Fix fsdax-mode namespace info-block zero-fields
>       libnvdimm/pfn: Stop padding pmem namespaces to section alignment
> 
> Robin Murphy (1):
>       mm/memremap: Rename and consolidate SECTION_SIZE
> 
> 
>  arch/powerpc/include/asm/sparsemem.h |    3 
>  arch/x86/mm/init_64.c                |    4 
>  drivers/nvdimm/dax_devs.c            |    2 
>  drivers/nvdimm/pfn.h                 |   15 -
>  drivers/nvdimm/pfn_devs.c            |   95 +++------
>  include/linux/memory_hotplug.h       |    7 -
>  include/linux/mm.h                   |    4 
>  include/linux/mmzone.h               |   93 +++++++--
>  kernel/memremap.c                    |   63 ++----
>  mm/hmm.c                             |    2 
>  mm/memory_hotplug.c                  |  172 +++++++++-------
>  mm/page_alloc.c                      |    8 -
>  mm/sparse-vmemmap.c                  |   21 +-
>  mm/sparse.c                          |  369 +++++++++++++++++++++++-----------
>  14 files changed, 511 insertions(+), 347 deletions(-)
> 

-- 
Sincerely yours,
Mike.

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