Re: [PATCH v1 4/4] arm64: drop pfn_valid_within() and simplify pfn_valid()

[David Hildenbrand <david@redhat.com>]

On 20.04.21 11:09, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> The arm64's version of pfn_valid() differs from the generic because of two
> reasons:
> 
> * Parts of the memory map are freed during boot. This makes it necessary to
>    verify that there is actual physical memory that corresponds to a pfn
>    which is done by querying memblock.
> 
> * There are NOMAP memory regions. These regions are not mapped in the
>    linear map and until the previous commit the struct pages representing
>    these areas had default values.
> 
> As the consequence of absence of the special treatment of NOMAP regions in
> the memory map it was necessary to use memblock_is_map_memory() in
> pfn_valid() and to have pfn_valid_within() aliased to pfn_valid() so that
> generic mm functionality would not treat a NOMAP page as a normal page.
> 
> Since the NOMAP regions are now marked as PageReserved(), pfn walkers and
> the rest of core mm will treat them as unusable memory and thus
> pfn_valid_within() is no longer required at all and can be disabled by
> removing CONFIG_HOLES_IN_ZONE on arm64.
> 
> pfn_valid() can be slightly simplified by replacing
> memblock_is_map_memory() with memblock_is_memory().
> 
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>   arch/arm64/Kconfig   | 3 ---
>   arch/arm64/mm/init.c | 4 ++--
>   2 files changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e4e1b6550115..58e439046d05 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1040,9 +1040,6 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
>   	def_bool y
>   	depends on NUMA
>   
> -config HOLES_IN_ZONE
> -	def_bool y
> -
>   source "kernel/Kconfig.hz"
>   
>   config ARCH_SPARSEMEM_ENABLE
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index c54e329aca15..370f33765b64 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -243,7 +243,7 @@ int pfn_valid(unsigned long pfn)
>   
>   	/*
>   	 * ZONE_DEVICE memory does not have the memblock entries.
> -	 * memblock_is_map_memory() check for ZONE_DEVICE based
> +	 * memblock_is_memory() check for ZONE_DEVICE based
>   	 * addresses will always fail. Even the normal hotplugged
>   	 * memory will never have MEMBLOCK_NOMAP flag set in their
>   	 * memblock entries. Skip memblock search for all non early
> @@ -254,7 +254,7 @@ int pfn_valid(unsigned long pfn)
>   		return pfn_section_valid(ms, pfn);
>   }
>   #endif
> -	return memblock_is_map_memory(addr);
> +	return memblock_is_memory(addr);
>   }
>   EXPORT_SYMBOL(pfn_valid);
>   
> 

What are the steps needed to get rid of custom pfn_valid() completely?

I'd assume we would have to stop freeing parts of the mem map during 
boot. How relevant is that for arm64 nowadays, especially with reduced 
section sizes?

-- 
Thanks,

David / dhildenb