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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > --- > [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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > [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.
