On Tue, Apr 20, 2021 at 11:22:53AM +0200, David Hildenbrand wrote:

On 20.04.21 11:09, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

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 <redacted>
Signed-off-by: Mike Rapoport <redacted>
---
  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.

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