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

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Message-ID: <ZyOUp5Juz5x3Ivrn@kernel.org>
Date: Thu, 31 Oct 2024 16:31:03 +0200
From: Mike Rapoport <rppt@...nel.org>
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, david@...hat.com, osalvador@...e.de,
	gregkh@...uxfoundation.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v4 1/3] memory: implement
 memory_block_advise/probe_max_size

On Tue, Oct 29, 2024 at 04:20:39PM -0400, 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;
> +static bool memory_block_advised_size_queried;

kernel-doc will be unhappy about variable declarations between the doc
block and the function it describes

> +int memory_block_advise_max_size(size_t size)
> +{
> +	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;
> +}
> +
>  unsigned long __weak memory_block_size_bytes(void)
>  {
>  	return MIN_MEMORY_BLOCK_SIZE;
> diff --git a/include/linux/memory.h b/include/linux/memory.h
> index c0afee5d126e..07e20a77b717 100644
> --- a/include/linux/memory.h
> +++ b/include/linux/memory.h
> @@ -149,6 +149,14 @@ static inline int hotplug_memory_notifier(notifier_fn_t fn, int pri)
>  {
>  	return 0;
>  }
> +static inline int memory_block_advise_max_size(size_t size)
> +{
> +	return -ENODEV;
> +}
> +static inline size_t memory_block_advised_max_size(void)
> +{
> +	return 0;
> +}
>  #else /* CONFIG_MEMORY_HOTPLUG */
>  extern int register_memory_notifier(struct notifier_block *nb);
>  extern void unregister_memory_notifier(struct notifier_block *nb);
> @@ -181,6 +189,8 @@ int walk_dynamic_memory_groups(int nid, walk_memory_groups_func_t func,
>  void memory_block_add_nid(struct memory_block *mem, int nid,
>  			  enum meminit_context context);
>  #endif /* CONFIG_NUMA */
> +int memory_block_advise_max_size(size_t size);
> +size_t memory_block_advised_max_size(void);
>  #endif	/* CONFIG_MEMORY_HOTPLUG */
>  
>  /*
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.