[PATCH v3 02/15] 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. It must
not be called for shared file-backed 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().

VMA userland tests are also updated accordingly.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 include/linux/mm.h              |  2 +-
 include/linux/pagemap.h         | 42 +++++++++++++++++++++++++++++++++++++++++
 tools/testing/vma/include/dup.h | 25 +++++++++++++++++++++++-
 3 files changed, 67 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index df78847f5f07..64214191e7c6 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..259177544b03 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1101,6 +1101,48 @@ static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
 	return pgoff;
 }
 
+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;
+}
+
+/**
+ * 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_test(vma, VMA_SHARED_BIT));
+	/* 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;
+}
+
 struct wait_page_key {
 	struct folio *folio;
 	int bit_nr;
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 26d9210b1e8b..5f1d93b3aefb 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1417,7 +1417,7 @@ 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;
 }
@@ -1610,3 +1610,26 @@ 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_test(vma, VMA_SHARED_BIT));
+	/* 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