/dev/zero, uniquely, marks memory mapped there as anonymous. This is
currently achieved using the mmap_action->success_hook.

However this hook circumvents the abstraction of VMA initialisation so it's
preferable to do things a different way.

To achieve this, this patch firstly defaults the VMA descriptor's vm_ops
field to the dummy VMA operations, which is what file-backed VMAs default
this field to.

That way, we can detect whether a driver sets this field to NULL in order
to mark it anonymous.

We then introduce vma_desc_set_anonymous() to do this explicitly, and
invoke it in mmap_zero_prepare().

This way, any driver which does not explicitly set desc->vm_ops, retains
the dummy vm_ops as they would previously.

We also update set_vma_user_defined_fields() to make clear that we are
either setting vma->vm_ops to what is provided by the driver (or defaulting
to dummy_vm_ops if not set), or setting the VMA anonymous.

This lays the groundwork for removing the success hook.

Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
---
 drivers/char/mem.c              | 17 +++++------------
 include/linux/mm.h              |  5 +++++
 mm/util.c                       |  1 +
 mm/vma.c                        |  3 +++
 tools/testing/vma/include/dup.h |  1 +
 5 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 5fd421e48c04..a4297eb39887 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -504,17 +504,6 @@ static ssize_t read_zero(struct file *file, char __user *buf,
 	return cleared;
 }
 
-static int mmap_zero_private_success(const struct vm_area_struct *vma)
-{
-	/*
-	 * This is a highly unique situation where we mark a MAP_PRIVATE mapping
-	 * of /dev/zero anonymous, despite it not being.
-	 */
-	vma_set_anonymous((struct vm_area_struct *)vma);
-
-	return 0;
-}
-
 static int mmap_zero_prepare(struct vm_area_desc *desc)
 {
 #ifndef CONFIG_MMU
@@ -523,7 +512,11 @@ static int mmap_zero_prepare(struct vm_area_desc *desc)
 	if (vma_desc_test(desc, VMA_SHARED_BIT))
 		return shmem_zero_setup_desc(desc);
 
-	desc->action.success_hook = mmap_zero_private_success;
+	/*
+	 * This is a highly unique situation where we mark a MAP_PRIVATE mapping
+	 * of /dev/zero anonymous, despite it not being.
+	 */
+	vma_desc_set_anonymous(desc);
 	return 0;
 }
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9cedc5e75aa9..2138c86403f5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1489,6 +1489,11 @@ static inline void vma_set_anonymous(struct vm_area_struct *vma)
 	vma->vm_ops = NULL;
 }
 
+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)
 {
 	return !vma->vm_ops;
diff --git a/mm/util.c b/mm/util.c
index 3cc949a0b7ed..2b2a9df689d7 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1192,6 +1192,7 @@ void compat_set_desc_from_vma(struct vm_area_desc *desc,
 	desc->vm_file = vma->vm_file;
 	desc->vma_flags = vma->flags;
 	desc->page_prot = vma->vm_page_prot;
+	desc->vm_ops = vma->vm_ops;
 
 	/* Default. */
 	desc->action.type = MMAP_NOTHING;
diff --git a/mm/vma.c b/mm/vma.c
index d90791b00a7b..9eea2850818a 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2697,6 +2697,8 @@ static void set_vma_user_defined_fields(struct vm_area_struct *vma,
 {
 	if (map->vm_ops)
 		vma->vm_ops = map->vm_ops;
+	else	/* Only /dev/zero should do this. */
+		vma_set_anonymous(vma);
 	vma->vm_private_data = map->vm_private_data;
 }
 
@@ -2744,6 +2746,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 		.action = {
 			.type = MMAP_NOTHING, /* Default to no further action. */
 		},
+		.vm_ops = &vma_dummy_vm_ops,
 	};
 	bool allocated_new = false;
 	int error;
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 9e0dfd3a85b0..306171d061e7 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1303,6 +1303,7 @@ static inline void compat_set_desc_from_vma(struct vm_area_desc *desc,
 	desc->vm_file = vma->vm_file;
 	desc->vma_flags = vma->flags;
 	desc->page_prot = vma->vm_page_prot;
+	desc->vm_ops = vma->vm_ops;
 
 	/* Default. */
 	desc->action.type = MMAP_NOTHING;
-- 
2.54.0