[PATCH v2 02/33] mm: add kdoc comments for vma_start/last_pgoff()

Describe what vma_start_pgoff() and vma_last_pgoff() actually provide in
detail.

This is in order that we can differentiate this between functions that will
be added in a subsequent patch which provide a different page offset.

We go to lengths to describe the edge cases that can be run into here.

No functional change intended.

Reviewed-by: Pedro Falcato <[email protected]>
Reviewed-by: Gregory Price <[email protected]>
Reviewed-by: Vlastimil Babka (SUSE) <[email protected]>
Signed-off-by: Lorenzo Stoakes <[email protected]>
---
 include/linux/mm.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 09b06d8fea74..abac72785277 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4307,11 +4307,41 @@ static inline unsigned long vma_pages(const struct vm_area_struct *vma)
 	return (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
 }
 
+/**
+ * vma_start_pgoff() - Get the page offset of the start of @vma
+ * @vma: The VMA whose page offset is required.
+ *
+ * If the VMA is file-backed, this is the page offset into the file.
+ *
+ * If @vma is anonymous, this is the virtual page offset of the start of the
+ * VMA - if unfaulted, then vma->vm_start >> PAGE_SHIFT, if faulted then the
+ * virtual page offset at the time of first fault.
+ *
+ * If @vma is a MAP_PRIVATE file-backed mapping, then this returns the
+ * page offset within the file.
+ *
+ * Edge cases: nommu does not abide by these, MAP_PRIVATE-/dev/zero satisfies
+ * vma_is_anonymous() but has file-backed page offset, and MAP_PRIVATE-pfnmap
+ * regions have their page offset set to the first PFN in the range.
+ *
+ * Returns: The page offset of the start of @vma.
+ */
 static inline pgoff_t vma_start_pgoff(const struct vm_area_struct *vma)
 {
 	return vma->vm_pgoff;
 }
 
+/**
+ * vma_last_pgoff() - Get the page offset of the last page in @vma
+ * @vma: The VMA whose last page offset is required.
+ *
+ * This returns the last page offset contained within @vma.
+ *
+ * See the description of vma_start_pgoff() for a description of VMA page
+ * offsets.
+ *
+ * Returns: The last page offset of @vma.
+ */
 static inline pgoff_t vma_last_pgoff(const struct vm_area_struct *vma)
 {
 	return vma_start_pgoff(vma) + vma_pages(vma) - 1;

-- 
2.55.0