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From:   Mike Rapoport <rppt@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        David Hildenbrand <david@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Will Deacon <will@kernel.org>, kvmarm@lists.cs.columbia.edu,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: [PATCH v4 1/4] include/linux/mmzone.h: add documentation for pfn_valid()
Date:   Tue, 11 May 2021 13:05:47 +0300
Message-Id: <20210511100550.28178-2-rppt@kernel.org>
In-Reply-To: <20210511100550.28178-1-rppt@kernel.org>
References: <20210511100550.28178-1-rppt@kernel.org>
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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>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 include/linux/mmzone.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0d53eba1c383..e5945ca24df7 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1427,6 +1427,17 @@ 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;
-- 
2.28.0