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

Mike Rapoport
2021-04-20T12:58:05Z

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.
*/

--
Sincerely yours,
Mike.