Re: [PATCH v2 1/3] mm/memblock: implement memblock_advise_size_order and probe functions

[Mike Rapoport <rppt@kernel.org>]

On Wed, Oct 16, 2024 at 03:24:43PM -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 as small as 256MB with a matching physical alignment.
> 
> Implement memblock_advise_size_order for use during early init, prior
> to allocator and smp init, for software to advise the system as to what
> the preferred block size should be.
> 
> The probe function is meant for arch_init code to fetch this value
> once during memblock size calculation. Use of the advisement value
> is arch-specific, and no guarantee is made that it will be used.

I'm confused.
 
Aren't we talking about memory blocks for hotplugable memory here?
This functionality rather belongs to drivers/base/memory.c, doesn't it?

> Calls to either function after probe results in -EBUSY to signal that
> advisement is ignored or that memblock_get_size_bytes should be used.
> 
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>  include/linux/memblock.h |  2 ++
>  mm/memblock.c            | 49 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+)
> 
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index fc4d75c6cec3..efb1f7cfbd58 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -111,6 +111,8 @@ static inline void memblock_discard(void) {}
>  #endif
>  
>  void memblock_allow_resize(void);
> +int memblock_advise_size_order(int order);
> +int memblock_probe_size_order(void);
>  int memblock_add_node(phys_addr_t base, phys_addr_t size, int nid,
>  		      enum memblock_flags flags);
>  int memblock_add(phys_addr_t base, phys_addr_t size);
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 3b9dc2d89b8a..e0bdba011564 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2009,6 +2009,55 @@ void __init memblock_allow_resize(void)
>  	memblock_can_resize = 1;
>  }
>  
> +/*
> + * @order: bit-order describing the preferred minimum block size
> + *
> + * Intended for use by early-boot software prior to smp and allocator init to
> + * advise the architecture what the minimum block size should be. Should only
> + * be called during arch init before allocator and smp init.
> + *
> + * This value can only decrease after it has been initially set, the intention
> + * is to identify the smallest supported alignment across all opinions.
> + *
> + * Use of this advisement value is arch-specific.
> + *
> + * Returns: 0 on success, -EINVAL if order is <=0, and -EBUSY if already probed
> + */
> +static int memblock_sz_order;
> +#define MEMBLOCK_SZO_PROBED (-1)
> +int memblock_advise_size_order(int order)
> +{
> +	if (order <= 0)
> +		return -EINVAL;
> +
> +	if (memblock_sz_order == MEMBLOCK_SZO_PROBED)
> +		return -EBUSY;
> +
> +	if (memblock_sz_order)
> +		memblock_sz_order = min(order, memblock_sz_order);
> +	else
> +		memblock_sz_order = order;
> +
> +	return 0;
> +}
> +
> +/*
> + * memblock_probe_size_order is intended for arch init code to probe one time,
> + * for a suggested memory block size.  After the first call, the result will
> + * always be -EBUSY. A late user should call memory_block_size_bytes instead to
> + * determine the actual block size in use.
> + *
> + * Should only be called during arch init prior to allocator and smp init.
> + *
> + * Returns: block size order, 0 if never set, or -EBUSY if previously probed.
> + */
> +int memblock_probe_size_order(void)
> +{
> +	int rv = xchg(&memblock_sz_order, -1);
> +
> +	return (rv == -1) ? -EBUSY : rv;
> +}
> +
>  static int __init early_memblock(char *p)
>  {
>  	if (p && strstr(p, "debug"))
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.