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

David Hildenbrand
2021-04-20T12:59:01Z

On 20.04.21 14:57, Mike Rapoport wrote:
> On Tue, Apr 20, 2021 at 11:22:53AM +0200, David Hildenbrand wrote:
>> 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.
>
> Well, I mean valid in the sense you can access the struct page :)
> How about:
>
> /**
> * pfn_valid - check if there is a memory map entry for a PFN
> * @pfn: the page frame number to check
> *
> * Check if there is a memory map entry aka struct page for the @pfn and it
> * is safe to access that struct page; the struct page state may be
> * uninitialized -- see pfn_to_online_page().
> *
> * 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.
> */
>

Sounds good to me -- thanks!

Reviewed-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb