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

[Gregory Price <gourry@gourry.net>]

On Mon, Oct 21, 2024 at 11:51:38AM +0200, David Hildenbrand wrote:
> 
> 
> 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().
>

If you have any links, happy to do some reading up on it.  Was going to look
into some more memblock behavior in the future so it's worth looking at.

> 
> 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.
> 

This is why I wanted to avoid a new interface in the first place and just
piggyback on set_memory_block_size_order - now there are two interfaces to
do the same thing and more hurdles.  But I suppose the suggestive-nature of
this one makes it far less offensive since it can be completely ignored.

~Gregory