[PATCH 15/30] mm: introduce and use linear_page_delta()

It's often useful to obtain the number of pages a given address lies at
within a VMA.

Add linear_page_delta() to determine this and update linear_page_index() to
make use of it.

Add comments to describe both linear_page_delta() and linear_page_index().

No functional change intended.

Signed-off-by: Lorenzo Stoakes <[email protected]>
---
 include/linux/pagemap.h | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2c3718d592d6..644c0f25ae73 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1063,11 +1063,44 @@ static inline pgoff_t folio_pgoff(const struct folio *folio)
 	return folio->index;
 }
 
+/**
+ * linear_page_delta() - Determine the relative page offset of @address within
+ * @vma.
+ * @vma: The VMA in which @address resides.
+ * @address: The address whose relative page offset is required.
+ *
+ * The result is identical for both file-backed and anonymous mappings and
+ * simply determines how many pages @address lies from @vma->vm_start.
+ *
+ * Returns: The number of pages @address is offset by within @vma.
+ */
+static inline pgoff_t linear_page_delta(const struct vm_area_struct *vma,
+					const unsigned long address)
+{
+	return (address - vma->vm_start) >> PAGE_SHIFT;
+}
+
+/**
+ * linear_page_index() - Determine the absolute page offset of @address within
+ * @vma.
+ * @vma: The VMA in which @address resides.
+ * @address: The address whose absolute page offset is required.
+ *
+ * For file-backed mappings, this returns the page offset of @address within the
+ * file.
+ *
+ * For anonymous mappings, this returns the virtual page offset of @address,
+ * which is the page offset the address possessed at the time the VMA was first
+ * faulted.
+ *
+ * Returns: The absolute page offset of @address within @vma.
+ */
 static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
 					const unsigned long address)
 {
 	pgoff_t pgoff;
-	pgoff = (address - vma->vm_start) >> PAGE_SHIFT;
+
+	pgoff = linear_page_delta(vma, address);
 	pgoff += vma->vm_pgoff;
 	return pgoff;
 }
@@ -1219,7 +1252,7 @@ static inline vm_fault_t folio_lock_or_retry(struct folio *folio,
 void folio_wait_bit(struct folio *folio, int bit_nr);
 int folio_wait_bit_killable(struct folio *folio, int bit_nr);
 
-/* 
+/*
  * Wait for a folio to be unlocked.
  *
  * This must be called with the caller "holding" the folio,
-- 
2.54.0