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

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Message-ID: <55df76a9-afa3-4dc0-a7f9-ff9b6f139448@redhat.com>
Date: Wed, 30 Oct 2024 11:25:33 +0100
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>, x86@...nel.org,
 linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org, linux-mm@...ck.org
Cc: 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 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.

> +static bool memory_block_advised_size_queried;
> +int memory_block_advise_max_size(size_t size)

Not that memory_block_size_bytes() uses "unsigned long". I don't think 
it matters here. Or could it on 32bit? (I assume that code will not 
really matter on 32bit)

> +{
> +	if (!size || !is_power_of_2(size))
> +		return -EINVAL;
> +
> +	if (memory_block_advised_size_queried)
> +		return -EBUSY;
> +
> +	if (memory_block_advised_sz)
> +		memory_block_advised_sz = min(size, memory_block_advised_sz);
> +	else
> +		memory_block_advised_sz = size;
> +
> +	return 0;
> +}
> +
> +/**
> + * memory_block_advised_max_size() - query advised max hotplug block size.
> + *
> + * After the first call, the value can never change. Callers looking for the
> + * actual block size should use memory_block_size_bytes. This interface is
> + * intended for use by arch-init when initializing the hotplug block size.
> + *
> + * Return: advised size in bytes, or 0 if never set.
> + */
> +size_t memory_block_advised_max_size(void)
> +{
> +	memory_block_advised_size_queried = true;
 > +	return memory_block_advised_sz;> +}
> +

I wonder if both should.could be "__init" ? So they could only be called 
from __init ... which sounds like the tight thing to do?

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb