linux-kernel - Re: [PATCH v8 0/3] memory,x86,acpi: hotplug memory alignment advisement

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Message-ID: <01d8cde7-0973-4303-bb5d-3d377a6862cb@redhat.com>
Date: Tue, 1 Apr 2025 21:08:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>, Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org, linux-acpi@...r.kernel.org, kernel-team@...a.com,
 x86@...nel.org, linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
 luto@...nel.org, peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, hpa@...or.com, rafael@...nel.org, lenb@...nel.org,
 gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
 dan.j.williams@...el.com, Jonathan.Cameron@...wei.com,
 alison.schofield@...el.com, rrichter@....com, rppt@...nel.org,
 bfaccini@...dia.com, haibo1.xu@...el.com, dave.jiang@...el.com,
 Ira Weiny <ira.weiny@...el.com>, Fan Ni <fan.ni@...sung.com>
Subject: Re: [PATCH v8 0/3] memory,x86,acpi: hotplug memory alignment
 advisement

On 01.04.25 20:53, Oscar Salvador wrote:
> On Mon, Jan 27, 2025 at 10:34:02AM -0500, Gregory Price wrote:
>> v8: nits and tag pickups
>>
>> 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 (i.e. not special purpose memory).
> 
> I wonder if something like this could help us in improving the
> ridiculous situation of having 16MB memory-block size on powerpc.

They have this granularity because ... they want to add/remove memory in 
16MiB on some powerpc dlpar machines :(

probe_memory_block_size() can query the hypervisor on the actual 
hot(un)plug size. IIRC, QEMU sets it to 256 MiB.

-- 
Cheers,

David / dhildenb