linux-kernel - Re: [PATCH v4 1/3] memory: implement memory_block_advise/probe_max

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Message-ID: <c99e0eca-49f6-4167-ad2b-d4168193ea7e@redhat.com>
Date: Wed, 30 Oct 2024 17:35:18 +0100
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
 linux-mm@...ck.org, linux-cxl@...r.kernel.org, Jonathan.Cameron@...wei.com,
 dan.j.williams@...el.com, rrichter@....com, Terry.Bowman@....com,
 dave.jiang@...el.com, ira.weiny@...el.com, alison.schofield@...el.com,
 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, osalvador@...e.de,
 gregkh@...uxfoundation.org, akpm@...ux-foundation.org, rppt@...nel.org
Subject: Re: [PATCH v4 1/3] memory: implement
 memory_block_advise/probe_max_size

On 30.10.24 15:59, Gregory Price wrote:
> On Wed, Oct 30, 2024 at 11:25:33AM +0100, David Hildenbrand wrote:
>> On 29.10.24 21:20, Gregory Price wrote:
>>> Hotplug memory sources may have opinions on what the memblock size
>>> should be - usually for alignment purposes.  For example, CXL memory
>>> extents can be 256MB with a matching alignment. If this size/alignment
>>> is smaller than the block size, it can result in stranded capacity.
>>>
>>> Implement memory_block_advise_max_size for use prior to allocator init,
>>> for software to advise the system on the max block size.
>>>
>>> Implement memory_block_probe_max_size for use by arch init code to
>>> calculate the best block size. Use of advice is architecture defined.
>>>
>>> The probe value can never change after first probe. Calls to advise
>>> after probe will return -EBUSY to aid debugging.
>>>
>>> On systems without hotplug, always return -ENODEV and 0 respectively.
>>>
>>> Suggested-by: Ira Weiny <ira.weiny@...el.com>
>>> Signed-off-by: Gregory Price <gourry@...rry.net>
>>> ---
>>>    drivers/base/memory.c  | 48 ++++++++++++++++++++++++++++++++++++++++++
>>>    include/linux/memory.h | 10 +++++++++
>>>    2 files changed, 58 insertions(+)
>>>
>>> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
>>> index 67858eeb92ed..099a972c52dc 100644
>>> --- a/drivers/base/memory.c
>>> +++ b/drivers/base/memory.c
>>> @@ -110,6 +110,54 @@ static void memory_block_release(struct device *dev)
>>>    	kfree(mem);
>>>    }
>>> +/**
>>> + * memory_block_advise_max_size() - advise memory hotplug on the max suggested
>>> + *				    block size, usually for alignment.
>>> + * @size: suggestion for maximum block size. must be aligned on power of 2.
>>> + *
>>> + * Early boot software (pre-allocator init) may advise archs on the max block
>>> + * size. This value can only decrease after initialization, as the intent is
>>> + * to identify the largest supported alignment for all sources.
>>> + *
>>> + * Use of this value is arch-defined, as is min/max block size.
>>> + *
>>> + * Return: 0 on success
>>> + *	   -EINVAL if size is 0 or not pow2 aligned
>>> + *	   -EBUSY if value has already been probed
>>> + */
>>> +static size_t memory_block_advised_sz;
>>
>> Nit: if everything is called "size", call this "size" as well.
>>
> 
> Mostly shortened here because
> 
> 	if (memory_block_advised_sz)
> 		memory_block_advised_size = min(size, memory_block_advised_size);
> 
> is over 80 characters lol.  Happy to change if you have strong feelings.

Feel free to exceed 80 chars if there is good reason to -- like in this 
case. checkpatch.pl nowadays complains if you exceed 100 chars.

No strong feelings, making it consistent in some way would be "nice" ;)

-- 
Cheers,

David / dhildenb