Re: [PATCH v1 1/4] include/linux/mmzone.h: add documentation for pfn_valid()

[David Hildenbrand <david@redhat.com>]

On 20.04.21 11:09, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> Add comment describing the semantics of pfn_valid() that clarifies that
> pfn_valid() only checks for availability of a memory map entry (i.e. struct
> page) for a PFN rather than availability of usable memory backing that PFN.
> 
> The most "generic" version of pfn_valid() used by the configurations with
> SPARSEMEM enabled resides in include/linux/mmzone.h so this is the most
> suitable place for documentation about semantics of pfn_valid().
> 
> Suggested-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>   include/linux/mmzone.h | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 47946cec7584..961f0eeefb62 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1410,6 +1410,17 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>   #endif
>   
>   #ifndef CONFIG_HAVE_ARCH_PFN_VALID
> +/**
> + * pfn_valid - check if there is a valid memory map entry for a PFN
> + * @pfn: the page frame number to check
> + *
> + * Check if there is a valid memory map entry aka struct page for the @pfn.
> + * Note, that availability of the memory map entry does not imply that
> + * there is actual usable memory at that @pfn. The struct page may
> + * represent a hole or an unusable page frame.
> + *
> + * Return: 1 for PFNs that have memory map entries and 0 otherwise
> + */
>   static inline int pfn_valid(unsigned long pfn)
>   {
>   	struct mem_section *ms;
> 

I'd rephrase all "there is a valid memory map" to "there is a memory 
map" and add "pfn_valid() does to indicate whether the memory map as 
actually initialized -- see pfn_to_online_page()."

pfn_valid() means that we can do a pfn_to_page() and don't get a fault 
when accessing the "struct page". It doesn't state anything about the 
content.

-- 
Thanks,

David / dhildenb