Am 16.10.24 um 21:24 schrieb Gregory Price:
When physical address regions are not aligned to memory block size,
the misaligned portion is lost (stranded capacity).
Block size (min/max/selected) is architecture defined. Most architectures
tend to use the minimum block size or some simplistic heurist. On x86,
memory block size increases up to 2GB, and is otherwise fitted to the
alignment of non-hotplug (special purpose memory).
CXL exposes its memory for management through the ACPI CEDT (CXL Early
Detection Table) in a field called the CXL Fixed Memory Window. Per
the CXL specification, this memory must be aligned to at least 256MB.
When a CFMW aligns on a size less than the block size, this causes a
loss of up to 2GB per CFMW on x86. It is not uncommon for CFMW to be
allocated per-device - though this behavior is BIOS defined.
This patch set provides 3 things:
1) implement advise/probe functions in mm/memblock.c to report/probe
architecture agnostic hotplug memory alignment advice.
2) update x86 memblock size logic to consider the hotplug advice
3) add code in acpi/numa/srat.c to report CFMW alignment advice
The advisement interfaces are design to be called during arch_init
code prior to allocator and smp_init. start_kernel will call these
through setup_arch() (via acpi and mm/init_64.c on x86), which occurs
prior to mm_core_init and smp_init - so no need for atomics.
There's an attempt to signal callers to advise() that probe has already
occurred, but this is predicated on the notion that probe() actually
occurs (which presently only happens on x86). This is to assist debugging
future users who may mistakenly call this after allocator or smp init.
Likewise, if probe() occurs more than once, we return -EBUSY to prevent
inconsistent values from being reported - i.e. this interaction should
happen exactly once, and all other behavior is an error / the probed
value should be acquired via memory_block_size_bytes() instead.
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
Just as a side note, a while ago there was a discussion about variable-sized
memory blocks -- essentially removing memory_block_size_bytes().
The main issue is that this would change /sys/devices/system/memory/ in ways it
could break existing user space. I believe there are other corner cases that are
a bit nasty to handle (e.g., removing parts of a larger memory block), but
likely it could be handled.
--
Cheers,
David / dhildenb
