[PATCH 2/2] mm: introduce vma_set_file function

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Add the new vma_set_file() function to allow changing
vma->vm_file with the necessary refcount dance.

Signed-off-by: Christian König <christian.koenig@amd.com>
---
 drivers/dma-buf/dma-buf.c | 16 +++++-----------
 include/linux/mm.h        |  2 ++
 mm/mmap.c                 | 16 ++++++++++++++++
 3 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 1699a8e309ef..672f3525ba74 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1163,20 +1163,14 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		return -EINVAL;
 
 	/* readjust the vma */
-	get_file(dmabuf->file);
-	oldfile = vma->vm_file;
-	vma->vm_file = dmabuf->file;
+	oldfile = vma_set_file(vma, dmabuf->file);
 	vma->vm_pgoff = pgoff;
 
 	ret = dmabuf->ops->mmap(dmabuf, vma);
-	if (ret) {
-		/* restore old parameters on failure */
-		vma->vm_file = oldfile;
-		fput(dmabuf->file);
-	} else {
-		if (oldfile)
-			fput(oldfile);
-	}
+	/* restore old parameters on failure */
+	if (ret)
+		vma_set_file(vma, oldfile);
+
 	return ret;
 
 }
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1983e08f5906..398a6fdaad1e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2688,6 +2688,8 @@ static inline void vma_set_page_prot(struct vm_area_struct *vma)
 }
 #endif
 
+struct file *vma_set_file(struct vm_area_struct *vma, struct file *file);
+
 #ifdef CONFIG_NUMA_BALANCING
 unsigned long change_prot_numa(struct vm_area_struct *vma,
 			unsigned long start, unsigned long end);
diff --git a/mm/mmap.c b/mm/mmap.c
index 40248d84ad5f..d3c3c510f643 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -136,6 +136,22 @@ void vma_set_page_prot(struct vm_area_struct *vma)
 	WRITE_ONCE(vma->vm_page_prot, vm_page_prot);
 }
 
+/*
+ * Change backing file, only valid to use during initial VMA setup.
+ */
+struct file *vma_set_file(struct vm_area_struct *vma, struct file *file)
+{
+	if (file)
+	        get_file(file);
+
+	swap(vma->vm_file, file);
+
+	if (file)
+		fput(file);
+
+	return file;
+}
+
 /*
  * Requires inode->i_mapping->i_mmap_rwsem
  */
-- 
2.17.1