Re: [PATCH v10 00/13] mm: Sub-section memory hotplug support

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Dan Williams <dan.j.williams@intel.com> writes:

> Changes since v9 [1]:
> - Fix multiple issues related to the fact that pfn_valid() has
>   traditionally returned true for any pfn in an 'early' (onlined at
>   boot) section regardless of whether that pfn represented 'System RAM'.
>   Teach pfn_valid() to maintain its traditional behavior in the presence
>   of subsections. Specifically, subsection precision for pfn_valid() is
>   only considered for non-early / hot-plugged sections. (Qian)
>
> - Related to the first item introduce a SECTION_IS_EARLY
>   (->section_mem_map flag) to remove the existing hacks for determining
>   an early section by looking at whether the usemap was allocated from the
>   slab.
>
> - Kill off the EEXIST hackery in __add_pages(). It breaks
>   (arch_add_memory() false-positive) the detection of subsection
>   collisions reported by section_activate(). It is also obviated by
>   David's recent reworks to move the 'System RAM' request_region() earlier
>   in the add_memory() sequence().
>
> - Switch to an arch-independent / static subsection-size of 2MB.
>   Otherwise, a per-arch subsection-size is a roadblock on the path to
>   persistent memory namespace compatibility across archs. (Jeff)
>
> - Update the changelog for "libnvdimm/pfn: Fix fsdax-mode namespace
>   info-block zero-fields" to clarify that the "Cc: stable" is only there
>   as safety measure for a distro that decides to backport "libnvdimm/pfn:
>   Stop padding pmem namespaces to section alignment", otherwise there is
>   no known bug exposure in older kernels. (Andrew)
>   
> - Drop some redundant subsection checks (Oscar)
>
> - Collect some reviewed-bys
>
> [1]: https://lore.kernel.org/lkml/155977186863.2443951.9036044808311959913.stgit@dwillia2-desk3.amr.corp.intel.com/


You can add Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
for ppc64.

BTW even after this series we have the kernel crash mentioned in the
below email on reconfigure. 

https://lore.kernel.org/linux-mm/20190514025354.9108-1-aneesh.kumar@linux.ibm.com

I guess we need to conclude how the reserve space struct page should be
initialized ?

>
> ---
>
> 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 a subsection-v10 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=subsection-v10
> [5]: https://github.com/pmem/ndctl/commit/7c59b4867e1c
>
> ---
>
> Dan Williams (13):
>       mm/sparsemem: Introduce struct mem_section_usage
>       mm/sparsemem: Introduce a SECTION_IS_EARLY flag
>       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: Document ZONE_DEVICE memory-model implications
>       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
>
>
>  Documentation/vm/memory-model.rst |   39 ++++
>  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            |   84 +++++++--
>  kernel/memremap.c                 |   61 +++----
>  mm/memory_hotplug.c               |  173 +++++++++----------
>  mm/page_alloc.c                   |   16 +-
>  mm/sparse-vmemmap.c               |   21 ++
>  mm/sparse.c                       |  335 ++++++++++++++++++++++++-------------
>  13 files changed, 494 insertions(+), 362 deletions(-)

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