drm/i915: Disable pagefaults along execbuffer relocation fast path

Along the fast path for relocation handling, we attempt to copy directly
from the user data structures whilst holding our mutex. This causes
lockdep to warn about circular lock dependencies if we need to pagefault
the user pages. [Since when handling a page fault on a mmapped bo, we
need to acquire the struct mutex whilst already holding the mm
semaphore, it is then verboten to acquire the mm semaphore when already
holding the struct mutex. The likelihood of the user passing in the
relocations contained in a GTT mmaped bo is low, but conceivable for
extreme pathology.] In order to force the mm to return EFAULT rather
than handle the pagefault, we therefore need to disable pagefaults
across the relocation fast path.

Signed-off-by: Chris Wilson <[e-mail]>
Cc: [e-mail]
Reviewed-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

···
 		uint32_t __iomem *reloc_entry;
 		void __iomem *reloc_page;
 
+		/* We can't wait for rendering with pagefaults disabled */
+		if (obj->active && in_atomic())
+			return -EFAULT;
+
 		ret = i915_gem_object_set_to_gtt_domain(obj, 1);
 		if (ret)
 			return ret;
···
 			     struct list_head *objects)
 {
 	struct drm_i915_gem_object *obj;
-	int ret;
+	int ret = 0;
 
+	/* This is the fast path and we cannot handle a pagefault whilst
+	 * holding the struct mutex lest the user pass in the relocations
+	 * contained within a mmaped bo. For in such a case we, the page
+	 * fault handler would call i915_gem_fault() and we would try to
+	 * acquire the struct mutex again. Obviously this is bad and so
+	 * lockdep complains vehemently.
+	 */
+	pagefault_disable();
 	list_for_each_entry(obj, objects, exec_list) {
 		ret = i915_gem_execbuffer_relocate_object(obj, eb);
 		if (ret)
-			return ret;
+			break;
 	}
+	pagefault_enable();
 
-	return 0;
+	return ret;
 }
 
 static int