[PATCH 1/3] drivers/char/mem: eliminate unnecessary use of success_hook

[Lorenzo Stoakes]

Introduce vma_desc_set_anonymous() and invoke it from mmap_zero_prepare()
in order to eliminate the use of the mmap_action->success_hook.

Note that file-backed VMAs default to vma->vm_ops == dummy_vm_ops, so if an
mmap or mmap_prepare hook does not set desc->vm_ops, the VMA will remain
file-backed.

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, which was provided
as a workaround for drivers (mostly, hugetlb) that still absolutely seemed
to rely upon having access to a VMA pointer.

However, for the transition to be useful, we really must eliminate this and
make no exceptions, otherwise drivers can simply work around it.

Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
---
 drivers/char/mem.c | 17 +++++------------
 include/linux/mm.h |  5 +++++
 mm/vma.c           |  2 ++
 3 files changed, 12 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/vma.c b/mm/vma.c
index d90791b00a7b..07486390c692 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;
 }
 
-- 
2.54.0