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().

We refer the reader to the comment for vma_start_pgoff() so we don't
duplicate information about the meaning of a VMA's page offset, which is
also less likely to result in bitrot.

No functional change intended.

Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-17-2a5aa403d977@kernel.org
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: SJ Park <sj@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Liam R. Howlett (Oracle) <liam@infradead.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 31 insertions, 2 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2c3718d592d6..a950070b045f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1063,11 +1063,40 @@ 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.
+ *
+ * See the comment for vma_start_pgoff() for a description of what the page
+ * offset signifies.
+ *
+ * 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 +1248,7 @@ 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,