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

[David Hildenbrand]

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@xxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxx>
> > > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxxx>

--
Thanks,

David / dhildenb