[076/192] include/linux/mmzone.h: add documentation for pfn_valid()

Commit Message

Andrew Morton July 1, 2021, 1:51 a.m. UTC

From: Mike Rapoport <[e-mail]>
Subject: include/linux/mmzone.h: add documentation for pfn_valid()

Patch series "arm64: drop pfn_valid_within() and simplify pfn_valid()", v4.

These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
pfn_valid_within() to 1.  

The idea is to mark NOMAP pages as reserved in the memory map and restore
the intended semantics of pfn_valid() to designate availability of struct
page for a pfn.

With this the core mm will be able to cope with the fact that it cannot
use NOMAP pages and the holes created by NOMAP ranges within MAX_ORDER
blocks will be treated correctly even without the need for
pfn_valid_within.


This patch (of 4):

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().

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Mike Rapoport <[e-mail]>
Suggested-by: Anshuman Khandual <[e-mail]>
Reviewed-by: Anshuman Khandual <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Reviewed-by: Kefeng Wang <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/mmzone.h |   11 +++++++++++
 1 file changed, 11 insertions(+)

Patch

--- a/include/linux/mmzone.h~include-linux-mmzoneh-add-documentation-for-pfn_valid
+++ a/include/linux/mmzone.h
@@ -1445,6 +1445,17 @@  static inline int pfn_section_valid(stru
 #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;