[PATCH v4 03/20] mm: introduce linear_anon_page_index()

[Lorenzo Stoakes (ARM)]

This function provides the anonymous equivalent of linear_page_index(),
instead offsetting based on the anonymous page offset of the VMA.

It is valid only for anonymous or MAP_PRIVATE file-backed mappings, in
other words CoW mappings.

For pure anon VMAs, this will be equal to linear_page_index().

Assert that both of these invariants are true In linear_anon_page_index()
and implement the algorithm in __linear_anon_page_index().

Note that MAP_PRIVATE-/dev/zero mappings will satisfy vma_is_anonymous()
but not fulfill this invariant, so when asserting this we check
vma->vm_file to account for this.

We do not update callsites yet, so no functional change intended.

Also const-ify vma_is_anonymous() to make it compatible with the
const-ified linear_anon_page_index().

While we're here, update linear_page_index() to be more succinct.

VMA userland tests are also updated accordingly.

Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 include/linux/mm.h              |  2 +-
 include/linux/pagemap.h         | 40 +++++++++++++++++++++++++++++++++++++---
 tools/testing/vma/include/dup.h | 25 ++++++++++++++++++++++++-
 3 files changed, 62 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a3368c542947..da36a6cc907c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1556,7 +1556,7 @@ static inline void vma_desc_set_anonymous(struct vm_area_desc *desc)
 	desc->vm_ops = NULL;
 }
 
-static inline bool vma_is_anonymous(struct vm_area_struct *vma)
+static inline bool vma_is_anonymous(const struct vm_area_struct *vma)
 {
 	return !vma->vm_ops;
 }
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index c6fc783aaee5..0adfa6605653 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1094,10 +1094,44 @@ static inline pgoff_t linear_page_delta(const struct vm_area_struct *vma,
 static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
 					const unsigned long address)
 {
-	pgoff_t pgoff;
+	return linear_page_delta(vma, address) + vma_start_pgoff(vma);
+}
+
+static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
+		const unsigned long address)
+{
+	return linear_page_delta(vma, address) + vma_start_anon_pgoff(vma);
+}
+
+/**
+ * linear_anon_page_index() - Determine the absolute anonymous page offset of
+ * @address within @vma.
+ * @vma: An anonymous or MAP_PRIVATE file-backed VMA in which @address resides.
+ * @address: The address whose absolute page offset is required.
+ *
+ * This returns the anonymous page offset of @address, which is the page offset
+ * the address possessed at the time the VMA was first faulted.
+ *
+ * For anonymous mappings, this returns the same value as linear_page_index().
+ *
+ * For MAP_PRIVATE file-backed mappings, this returns the anonymous page offset
+ * of @address, which is the page offset the address possessed at the time the
+ * VMA was first faulted.
+ *
+ * It is not valid to call this function for shared file-backed mappings.
+ *
+ * Returns: The absolute anonymous page offset of @address within @vma.
+ */
+static inline pgoff_t linear_anon_page_index(const struct vm_area_struct *vma,
+		const unsigned long address)
+{
+	const pgoff_t pgoff = __linear_anon_page_index(vma, address);
+
+	VM_WARN_ON_ONCE(!vma_is_cow_mapping(vma));
+	/* Account for MAP_PRIVATE-/dev/zero which is only semi-anonymous. */
+	if (vma_is_anonymous(vma) && !vma->vm_file)
+		VM_WARN_ON_ONCE(pgoff != linear_page_index(vma, address));
 
-	pgoff = linear_page_delta(vma, address);
-	pgoff += vma_start_pgoff(vma);
 	return pgoff;
 }
 
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 40ad83936b28..4c58487b764e 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1428,7 +1428,7 @@ static inline void vma_iter_set(struct vma_iterator *vmi, unsigned long addr)
 	mas_set(&vmi->mas, addr);
 }
 
-static inline bool vma_is_anonymous(struct vm_area_struct *vma)
+static inline bool vma_is_anonymous(const struct vm_area_struct *vma)
 {
 	return !vma->vm_ops;
 }
@@ -1621,3 +1621,26 @@ static inline pgprot_t vma_get_page_prot(const struct vm_area_struct *vma)
 {
 	return vma_flags_to_page_prot(vma->flags);
 }
+
+static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
+					       const unsigned long address)
+{
+	pgoff_t pgoff;
+
+	pgoff = linear_page_delta(vma, address);
+	pgoff += vma_start_anon_pgoff(vma);
+	return pgoff;
+}
+
+static inline pgoff_t linear_anon_page_index(const struct vm_area_struct *vma,
+		const unsigned long address)
+{
+	const pgoff_t pgoff = __linear_anon_page_index(vma, address);
+
+	VM_WARN_ON_ONCE(!vma_is_cow_mapping(vma));
+	/* Account for MAP_PRIVATE-/dev/zero which is only semi-anonymous. */
+	if (vma_is_anonymous(vma) && !vma->vm_file)
+		VM_WARN_ON_ONCE(pgoff != linear_page_index(vma, address));
+
+	return pgoff;
+}

-- 
2.55.0