Re: [PATCH 1/3] mm/memory_hotplug: Allow an override for the memmap_on_memory param

["Huang, Ying" <ying.huang@intel.com>]

Hi, Vishal,

Thanks for your patch!

Vishal Verma <vishal.l.verma@intel.com> writes:

> For memory hotplug to consider MHP_MEMMAP_ON_MEMORY behavior, the
> 'memmap_on_memory' module parameter was a hard requirement.
>
> In preparation for the dax/kmem driver to use memmap_on_memory
> semantics, arrange for the module parameter check to be bypassed via the
> appropriate mhp_flag.
>
> Recall that the kmem driver could contribute huge amounts of hotplugged
> memory originating from special purposes devices such as CXL memory
> expanders. In some cases memmap_on_memory may be the /only/ way this new
> memory can be hotplugged. Hence it makes sense for kmem to have a way to
> force memmap_on_memory without depending on a module param, if all the
> other conditions for it are met.
>
> The only other user of this interface is acpi/acpi_memoryhotplug.c,
> which only enables the mhp_flag if an initial
> mhp_supports_memmap_on_memory() test passes. Maintain the existing
> behavior and semantics for this by performing the initial check from
> acpi without the MHP_MEMMAP_ON_MEMORY flag, so its decision falls back
> to the module parameter.
>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Len Brown <lenb@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
> ---
>  include/linux/memory_hotplug.h |  2 +-
>  drivers/acpi/acpi_memhotplug.c |  2 +-
>  mm/memory_hotplug.c            | 24 ++++++++++++++++--------
>  3 files changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 9fcbf5706595..c9ddcd3cad70 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -358,7 +358,7 @@ extern struct zone *zone_for_pfn_range(int online_type, int nid,
>  extern int arch_create_linear_mapping(int nid, u64 start, u64 size,
>  				      struct mhp_params *params);
>  void arch_remove_linear_mapping(u64 start, u64 size);
> -extern bool mhp_supports_memmap_on_memory(unsigned long size);
> +extern bool mhp_supports_memmap_on_memory(unsigned long size, mhp_t mhp_flags);
>  #endif /* CONFIG_MEMORY_HOTPLUG */
>  
>  #endif /* __LINUX_MEMORY_HOTPLUG_H */
> diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c
> index 24f662d8bd39..119d3bb49753 100644
> --- a/drivers/acpi/acpi_memhotplug.c
> +++ b/drivers/acpi/acpi_memhotplug.c
> @@ -211,7 +211,7 @@ static int acpi_memory_enable_device(struct acpi_memory_device *mem_device)
>  		if (!info->length)
>  			continue;
>  
> -		if (mhp_supports_memmap_on_memory(info->length))
> +		if (mhp_supports_memmap_on_memory(info->length, 0))
>  			mhp_flags |= MHP_MEMMAP_ON_MEMORY;
>  		result = __add_memory(mgid, info->start_addr, info->length,
>  				      mhp_flags);
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 8e0fa209d533..bb3845830922 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1283,15 +1283,21 @@ static int online_memory_block(struct memory_block *mem, void *arg)
>  	return device_online(&mem->dev);
>  }
>  
> -bool mhp_supports_memmap_on_memory(unsigned long size)
> +bool mhp_supports_memmap_on_memory(unsigned long size, mhp_t mhp_flags)
>  {
>  	unsigned long nr_vmemmap_pages = size / PAGE_SIZE;
>  	unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
>  	unsigned long remaining_size = size - vmemmap_size;
>  
>  	/*
> -	 * Besides having arch support and the feature enabled at runtime, we
> -	 * need a few more assumptions to hold true:
> +	 * The MHP_MEMMAP_ON_MEMORY flag indicates a caller that wants to force
> +	 * memmap_on_memory (if other conditions are met), regardless of the
> +	 * module parameter. drivers/dax/kmem.c is an example, where large
> +	 * amounts of hotplug memory may come from, and the only option to
> +	 * successfully online all of it is to place the memmap on this memory.
> +	 *
> +	 * Besides having arch support and the feature enabled at runtime or
> +	 * via the mhp_flag, we need a few more assumptions to hold true:
>  	 *
>  	 * a) We span a single memory block: memory onlining/offlinin;g happens
>  	 *    in memory block granularity. We don't want the vmemmap of online
> @@ -1315,10 +1321,12 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
>  	 *       altmap as an alternative source of memory, and we do not exactly
>  	 *       populate a single PMD.
>  	 */
> -	return mhp_memmap_on_memory() &&
> -	       size == memory_block_size_bytes() &&
> -	       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> -	       IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT));
> +
> +	if ((mhp_flags & MHP_MEMMAP_ON_MEMORY) || mhp_memmap_on_memory())
> +		return size == memory_block_size_bytes() &&
> +		       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> +		       IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT));
> +	return false;
>  }
>  
>  /*
> @@ -1375,7 +1383,7 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
>  	 * Self hosted memmap array
>  	 */
>  	if (mhp_flags & MHP_MEMMAP_ON_MEMORY) {
> -		if (!mhp_supports_memmap_on_memory(size)) {
> +		if (!mhp_supports_memmap_on_memory(size, mhp_flags)) {
>  			ret = -EINVAL;
>  			goto error;
>  		}

It appears that we need to deal with the hot-remove path too.
try_remove_memory() will call mhp_memmap_on_memory() and only work with
MHP_MEMMAP_ON_MEMORY properly if it returns true.

Best Regards,
Huang, Ying