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Subject[RFC PATCH v2 0/4] Allocate memmap from hotadded memory (per device)
This is v2 of [1]:

Changes from v1 -> v2:
- 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.

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

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. Once the memmap is
allocated we need a way to mark altmap pfns used for the allocation.
If MHP_MEMMAP_ON_MEMORY flag was passed, we set up the layout of the
altmap structure in add_memory_resource), and then we call
mark_vmemmap_pages() to mark vmemmap pages.

memory_block gained a new field called nr_vmemmap_pages.
This plays well for two reasons:

1) {offline/online}_pages know the differente between start_pfn and
valid_start_pfn, which is start_pfn + nr_vmemmap_pages.
In this way all isolation/migration/initialization operations are
done to the right range of memory without vmemmap pages to get involved.
This allows us for a much cleaner handling.

2) In try_remove_memory, we construct a new vmemap_altmap struct with the
right info, so we end up calling vmem_altmap_free instead of free_pagetable
when removing the memory.


Oscar Salvador (4):
mm,memory_hotplug: Introduce MHP_MEMMAP_ON_MEMORY
mm,memory_hotplug: Allocate memmap from the added memory range
mm,memory_hotplug: Add mhp_supports_memmap_on_memory
mm,memory_hotplug: Enable MHP_MEMMAP_ON_MEMORY when supported

arch/arm64/mm/mmu.c | 5 +
arch/powerpc/mm/mem.c | 5 +
arch/powerpc/platforms/powernv/memtrace.c | 2 +-
arch/x86/mm/init_64.c | 5 +
drivers/acpi/acpi_memhotplug.c | 5 +-
drivers/base/memory.c | 21 ++--
include/linux/memory.h | 7 +-
include/linux/memory_hotplug.h | 21 +++-
include/linux/memremap.h | 2 +-
mm/memory_hotplug.c | 119 ++++++++++++++++++----
mm/page_alloc.c | 4 +-
11 files changed, 158 insertions(+), 38 deletions(-)


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