mm: introduce vma_set_file function v4

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

v2: add more users of this.
v3: add missing EXPORT_SYMBOL, rebase on mmap cleanup,
    add comments why we drop the reference on two occasions.
v4: make it clear that changing an anonymous vma is illegal.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/394773/

drivers/dma-buf/dma-buf.c

@@ -1183,8 +1183,7 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		return -EINVAL;
 
 	/* readjust the vma */
-	fput(vma->vm_file);
-	vma->vm_file = get_file(dmabuf->file);
+	vma_set_file(vma, dmabuf->file);
 	vma->vm_pgoff = pgoff;
 
 	return dmabuf->ops->mmap(dmabuf, vma);

drivers/gpu/drm/etnaviv/etnaviv_gem.c

@@ -145,10 +145,8 @@ static int etnaviv_gem_mmap_obj(struct etnaviv_gem_object *etnaviv_obj,
 		 * address_space (so unmap_mapping_range does what we want,
 		 * in particular in the case of mmap'd dmabufs)
 		 */
-		fput(vma->vm_file);
-		get_file(etnaviv_obj->base.filp);
 		vma->vm_pgoff = 0;
-		vma->vm_file  = etnaviv_obj->base.filp;
+		vma_set_file(vma, etnaviv_obj->base.filp);
 
 		vma->vm_page_prot = vm_page_prot;
 	}

drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c

@@ -114,8 +114,7 @@ static int i915_gem_dmabuf_mmap(struct dma_buf *dma_buf, struct vm_area_struct *
 	if (ret)
 		return ret;
 
-	fput(vma->vm_file);
-	vma->vm_file = get_file(obj->base.filp);
+	vma_set_file(vma, obj->base.filp);
 
 	return 0;
 }

drivers/gpu/drm/i915/gem/i915_gem_mman.c

@@ -893,8 +893,9 @@ int i915_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 	 * requires avoiding extraneous references to their filp, hence why
 	 * we prefer to use an anonymous file for their mmaps.
 	 */
-	fput(vma->vm_file);
-	vma->vm_file = anon;
+	vma_set_file(vma, anon);
+	/* Drop the initial creation reference, the vma is now holding one. */
+	fput(anon);
 
 	switch (mmo->mmap_type) {
 	case I915_MMAP_TYPE_WC:

drivers/gpu/drm/msm/msm_gem.c

@@ -212,10 +212,8 @@ int msm_gem_mmap_obj(struct drm_gem_object *obj,
 		 * address_space (so unmap_mapping_range does what we want,
 		 * in particular in the case of mmap'd dmabufs)
 		 */
-		fput(vma->vm_file);
-		get_file(obj->filp);
 		vma->vm_pgoff = 0;
-		vma->vm_file  = obj->filp;
+		vma_set_file(vma, obj->filp);
 
 		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	}

drivers/gpu/drm/omapdrm/omap_gem.c

@@ -564,9 +564,8 @@ int omap_gem_mmap_obj(struct drm_gem_object *obj,
 		 * address_space (so unmap_mapping_range does what we want,
 		 * in particular in the case of mmap'd dmabufs)
 		 */
-		fput(vma->vm_file);
 		vma->vm_pgoff = 0;
-		vma->vm_file  = get_file(obj->filp);
+		vma_set_file(vma, obj->filp);
 
 		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	}

drivers/gpu/drm/vgem/vgem_drv.c

@@ -397,8 +397,7 @@ static int vgem_prime_mmap(struct drm_gem_object *obj,
 	if (ret)
 		return ret;
 
-	fput(vma->vm_file);
-	vma->vm_file = get_file(obj->filp);
+	vma_set_file(vma, obj->filp);
 	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
 

drivers/staging/android/ashmem.c

@@ -450,9 +450,9 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
 		vma_set_anonymous(vma);
 	}
 
-	if (vma->vm_file)
-		fput(vma->vm_file);
-	vma->vm_file = asma->file;
+	vma_set_file(vma, asma->file);
+	/* XXX: merge this with the get_file() above if possible */
+	fput(asma->file);
 
 out:
 	mutex_unlock(&ashmem_mutex);

include/linux/mm.h

@@ -2719,6 +2719,8 @@ static inline void vma_set_page_prot(struct vm_area_struct *vma)
 }
 #endif
 
+void 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);

mm/mmap.c

@@ -136,6 +136,18 @@ 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.
+ */
+void vma_set_file(struct vm_area_struct *vma, struct file *file)
+{
+	/* Changing an anonymous vma with this is illegal */
+	get_file(file);
+	swap(vma->vm_file, file);
+	fput(file);
+}
+EXPORT_SYMBOL(vma_set_file);
+
 /*
  * Requires inode->i_mapping->i_mmap_rwsem
  */