Re: [PATCH v2 0/3] mm/memblock,x86,acpi: hotplug memory alignment advisement

[David Hildenbrand <david@redhat.com>]

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@intel.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Gregory Price <gourry@gourry.net>

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