[RFC PATCH 02/10] mm: introduce linear_virt_page_index()

[Lorenzo Stoakes]

This function provides the virtual equivalent of linear_page_index(),
instead offsetting based on the virtual 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().

We implement the algorithm in __linear_virt_page_index(), which is provided
for internal mm code that might be interacting with shared VMAs.

In linear_virt_page_index() we assert that both of these invariants are
true.

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_virt_page_index().

VMA userland tests are updated accordingly.

Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
---
 include/linux/mm.h              |  2 +-
 include/linux/pagemap.h         | 41 +++++++++++++++++++++++++++++++++
 tools/testing/vma/include/dup.h | 22 ++++++++++++++++++
 3 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index cd826c052be1..9451aa537abb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1533,7 +1533,7 @@ 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 68a88d34a468..6e0d719d639a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1105,6 +1105,47 @@ static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
 	return pgoff;
 }
 
+static inline pgoff_t __linear_virt_page_index(const struct vm_area_struct *vma,
+					       const unsigned long address)
+{
+	pgoff_t pgoff;
+
+	pgoff = linear_page_delta(vma, address);
+	pgoff += vma_start_virt_pgoff(vma);
+	return pgoff;
+}
+
+/**
+ * linear_virt_page_index() - Determine the absolute virtual 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 virtual 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 virtual 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 virtual page offset of @address within @vma.
+ */
+static inline pgoff_t linear_virt_page_index(const struct vm_area_struct *vma,
+					     const unsigned long address)
+{
+	const pgoff_t pgoff = __linear_virt_page_index(vma, address);
+
+	VM_WARN_ON_ONCE(vma_test(vma, VMA_SHARED_BIT));
+	if (!vma->vm_file) /* Is anonymous except MAP_PRIVATE-/dev/zero */
+		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 09cfbf9572e8..bac6caf2eaa2 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1601,3 +1601,25 @@ static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
 	pgoff += vma_start_pgoff(vma);
 	return pgoff;
 }
+
+static inline pgoff_t __linear_virt_page_index(const struct vm_area_struct *vma,
+					       const unsigned long address)
+{
+	pgoff_t pgoff;
+
+	pgoff = linear_page_delta(vma, address);
+	pgoff += vma_start_virt_pgoff(vma);
+	return pgoff;
+}
+
+static inline pgoff_t linear_virt_page_index(const struct vm_area_struct *vma,
+					     const unsigned long address)
+{
+	const pgoff_t pgoff = __linear_virt_page_index(vma, address);
+
+	VM_WARN_ON_ONCE(vma_test(vma, VMA_SHARED_BIT));
+	if (!vma->vm_file) /* Is anonymous except MAP_PRIVATE-/dev/zero */
+		VM_WARN_ON_ONCE(pgoff != linear_page_index(vma, address));
+
+	return pgoff;
+}
-- 
2.54.0