linux-kernel - Re: [PATCH v1 1/4] include/linux/mmzone.h: add documentation for pfn

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Message-ID: <07216f89-369f-0dcc-7bc0-97c204981396@redhat.com>
Date:   Tue, 20 Apr 2021 14:58:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 1/4] include/linux/mmzone.h: add documentation for
 pfn_valid()

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@...ux.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@....com>
>>> Signed-off-by: Mike Rapoport <rppt@...ux.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@...hat.com>

-- 
Thanks,

David / dhildenb