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/

10 files changed, 26 insertions, 19 deletions

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 3d22dff1dab9..696b4ce4cfd9 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/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);
diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
index 67d9a2b9ea6a..4132acfa11be 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
+++ b/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;
 	}
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
index 0dd477e56573..04e9c04545ad 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
+++ b/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;
 }
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index 3d69e51f3e4d..ec28a6cde49b 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/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:
diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index 2e1bce7c0b19..311721ceee50 100644
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/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);
 	}
diff --git a/drivers/gpu/drm/omapdrm/omap_gem.c b/drivers/gpu/drm/omapdrm/omap_gem.c
index d8e09792793a..f063f5a04fb0 100644
--- a/drivers/gpu/drm/omapdrm/omap_gem.c
+++ b/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);
 	}
diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
index fa54a6d1403d..ea0eecae5153 100644
--- a/drivers/gpu/drm/vgem/vgem_drv.c
+++ b/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));
 
diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 10b4be1f3e78..4789d36ddfd3 100644
--- a/drivers/staging/android/ashmem.c
+++ b/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);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef360fe70aaf..2b7ac36c42dd 100644
--- a/include/linux/mm.h
+++ b/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);
diff --git a/mm/mmap.c b/mm/mmap.c
index 30a4e8412a58..b0093cc69240 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -137,6 +137,18 @@ void vma_set_page_prot(struct vm_area_struct *vma)
 }
 
 /*
+ * 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
  */
 static void __remove_shared_vm_struct(struct vm_area_struct *vma,