[Intel-gfx] [CI 2/2] drm/i915/gem: Drop local vma->vm_file reference

[Chris Wilson]

We use the global device inode, shared amongst all files, and not the
user's device filp to provide the backing storage for the mmap. The
vma->vm_file provides a redundant reference that breaks existing
expected behaviour that closing the user's device fd will release the
resources bound to it, if a mmap persists. (Even without the
vma->vm_file, the mmap will persist past the user's fd as the storage is
bound to the device, i.e. our reference is on the object not file.)

Fixes: cc662126b413 ("drm/i915: Introduce DRM_I915_GEM_MMAP_OFFSET")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/919
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gem/i915_gem_mman.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index 905527ce2999..20f065e02fe2 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
@@ -699,6 +699,7 @@ int i915_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 	struct drm_device *dev = priv->minor->dev;
 	struct i915_mmap_offset *mmo = NULL;
 	struct drm_gem_object *obj = NULL;
+	struct file *anon;
 
 	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
@@ -747,9 +748,26 @@ int i915_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 		vma->vm_flags &= ~VM_MAYWRITE;
 	}
 
+	anon = drm_minor_create_anonfile(dev->primary, vma->vm_file->f_flags);
+	if (IS_ERR(anon)) {
+		drm_gem_object_put_unlocked(obj);
+		return PTR_ERR(anon);
+	}
+
 	vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_private_data = mmo;
 
+	/*
+	 * We keep the ref on mmo->obj, not vm_file, but we require
+	 * vma->vm_file->f_mapping, see vma_link(), for later revocation.
+	 * Our userspace is accustomed to having per-file resource cleanup
+	 * (i.e. contexts, objects and requests) on their close(fd), which
+	 * 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;
+
 	switch (mmo->mmap_type) {
 	case I915_MMAP_TYPE_WC:
 		vma->vm_page_prot =
-- 
2.25.0.rc0