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Subject[PATCH v5 0/5] Allocate memmap from hotadded memory (per device)
Changes from v4 -> v5:
- Addressed feedback from David (patch#1)
- Tested on x86_64 with different struct page sizes and on large/small memory
- Tested on arm64 with 4K, 64K (with and without THP) and with different struct
page sizes

NOTE: We might need to make this feature and hugetlb-vmemmap feature [1] mutually
exclusive. I raised an issue I see in [2].
Hugetlb-vmemmap feature has been withdrawn for the time being due to the need
in further changes wrt. locking/freeing context.
I will keep an eye, and when the time comes again I will see how the two
features play together and how one another can be disabled when needed.

Changes from v3 -> v4:
- Addressed feedback from David
- Wrap memmap_on_memory module thingy with #ifdef
in generic mm/Kconfig
- Collect David's Reviewed-bys

Changes from v2 -> v3:
- Addressed feedback from David
- Squash former patch#4 and and patch#5 into patch#1
- Simplify module parameter functions

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.


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.
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 (5):
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

Documentation/admin-guide/kernel-parameters.txt | 16 +++
arch/arm64/Kconfig | 3 +
arch/x86/Kconfig | 3 +
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 | 5 +
mm/Makefile | 5 +-
mm/memory_hotplug.c | 166 +++++++++++++++++++++---
mm/page_alloc.c | 4 +-
13 files changed, 225 insertions(+), 38 deletions(-)


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