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    SubjectRe: [PATCH v2 0/7] Allocate memmap from hotadded memory (per device)
    On Tue, Feb 09, 2021 at 02:38:47PM +0100, Oscar Salvador wrote:
    > Hi,
    >
    > here is v2.
    >
    > Changes from v1 -> v2
    > - Addressed feedback from David
    > - Fence off the feature in case struct page size is not
    > multiple of PMD size or pageblock alignment cannot be guaranted
    > - Tested on x86_64 small and large memory_blocks
    > - Tested on arm64 4KB and 64KB page sizes (for some reason I cannot boot
    > my VM with 16KB page size).
    >
    > Arm64 with 4KB page size behaves like x86_64 after [1], which made section
    > size smaller.
    > With 64KB, the feature gets fenced off due to pageblock alignment.
    >
    > Changes from RFCv3 -> v1:
    > - Addressed feedback from David
    > - Re-order patches
    >
    > Changes from v2 -> v3 (RFC):
    > - Re-order patches (Michal)
    > - Fold "mm,memory_hotplug: Introduce MHP_MEMMAP_ON_MEMORY" in patch#1
    > - Add kernel boot option to enable this feature (Michal)
    >
    > Changes from v1 -> v2 (RFC):
    > - Addressed feedback provided by David
    > - Add a arch_support_memmap_on_memory to be called
    > from mhp_supports_memmap_on_memory, as atm,
    > only ARM, powerpc and x86_64 have altmat support.
    >
    > [1] https://lore.kernel.org/lkml/cover.1611206601.git.sudaraja@codeaurora.org/

    Let me refloat this one :-)

    >
    > Original cover letter:
    >
    > ----
    >
    > The primary goal of this patchset is to reduce memory overhead of the
    > hot-added memory (at least for SPARSEMEM_VMEMMAP memory model).
    > The current way we use to populate memmap (struct page array) has two main drawbacks:
    >
    > a) it consumes an additional memory until the hotadded memory itself is
    > onlined and
    > b) memmap might end up on a different numa node which is especially true
    > for movable_node configuration.
    > c) due to fragmentation we might end up populating memmap with base
    > pages
    >
    > One way to mitigate all these issues is to simply allocate memmap array
    > (which is the largest memory footprint of the physical memory hotplug)
    > from the hot-added memory itself. SPARSEMEM_VMEMMAP memory model allows
    > us to map any pfn range so the memory doesn't need to be online to be
    > usable for the array. See patch 3 for more details.
    > This feature is only usable when CONFIG_SPARSEMEM_VMEMMAP is set.
    >
    > [Overall design]:
    >
    > Implementation wise we reuse vmem_altmap infrastructure to override
    > the default allocator used by vmemap_populate.
    > memory_block structure gained a new field called nr_vmemmap_pages.
    > This plays well for two reasons:
    >
    > 1) {offline/online}_pages know the difference between start_pfn and
    > buddy_start_pfn, which is start_pfn + nr_vmemmap_pages.
    > In this way all isolation/migration operations are
    > done to within the right range of memory without vmemmap pages.
    > This allows us for a much cleaner handling.
    >
    > 2) In try_remove_memory, we construct a new vmemap_altmap struct with the
    > right information based on memory_block->nr_vmemap_pages, so we end up
    > calling vmem_altmap_free instead of free_pagetable when removing the memory.
    >
    > Oscar Salvador (7):
    > mm,memory_hotplug: Allocate memmap from the added memory range
    > acpi,memhotplug: Enable MHP_MEMMAP_ON_MEMORY when supported
    > mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory
    > mm,memory_hotplug: Enforce struct page size to be multiple of PMD
    > mm,memory_hotplug: Enforce pageblock alignment when memmap_on_memory
    > x86/Kconfig: Introduce ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
    > arm64/Kconfig: Introduce ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
    >
    > Documentation/admin-guide/kernel-parameters.txt | 14 +++
    > arch/arm64/Kconfig | 4 +
    > arch/x86/Kconfig | 4 +
    > drivers/acpi/acpi_memhotplug.c | 5 +-
    > drivers/base/memory.c | 20 ++--
    > include/linux/memory.h | 8 +-
    > include/linux/memory_hotplug.h | 21 +++-
    > include/linux/memremap.h | 2 +-
    > include/linux/mmzone.h | 5 +
    > mm/Kconfig | 3 +
    > mm/Makefile | 5 +-
    > mm/memory_hotplug.c | 150 ++++++++++++++++++++----
    > mm/page_alloc.c | 4 +-
    > 13 files changed, 208 insertions(+), 37 deletions(-)
    >
    > --
    > 2.16.3
    >
    >

    --
    Oscar Salvador
    SUSE L3

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