drm/i915: Introduce i915_address_space.mutex

Add a mutex into struct i915_address_space to be used while operating on
the vma and their lists for a particular vm. As this may be called from
the shrinker, we taint the mutex with fs_reclaim so that from the start
lockdep warns us if we are caught holding the mutex across an
allocation. (With such small steps we will eventually rid ourselves of
struct_mutex recursion!)

Signed-off-by: Chris Wilson <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/i915_drv.h

···
 unsigned long i915_gem_shrink_all(struct drm_i915_private *i915);
 void i915_gem_shrinker_register(struct drm_i915_private *i915);
 void i915_gem_shrinker_unregister(struct drm_i915_private *i915);
-
+void i915_gem_shrinker_taints_mutex(struct mutex *mutex);
 
 /* i915_gem_tiling.c */
 static inline bool i915_gem_object_needs_bit17_swizzle(struct drm_i915_gem_object *obj)

drivers/gpu/drm/i915/i915_gem_gtt.c

···
 static void i915_address_space_init(struct i915_address_space *vm,
 				    struct drm_i915_private *dev_priv)
 {
+	/*
+	 * The vm->mutex must be reclaim safe (for use in the shrinker).
+	 * Do a dummy acquire now under fs_reclaim so that any allocation
+	 * attempt holding the lock is immediately reported by lockdep.
+	 */
+	mutex_init(&vm->mutex);
+	i915_gem_shrinker_taints_mutex(&vm->mutex);
+
 	GEM_BUG_ON(!vm->total);
 	drm_mm_init(&vm->mm, 0, vm->total);
 	vm->mm.head_node.color = I915_COLOR_UNEVICTABLE;
···
 	spin_unlock(&vm->free_pages.lock);
 
 	drm_mm_takedown(&vm->mm);
+
+	mutex_destroy(&vm->mutex);
 }
 
 static int __setup_page_dma(struct i915_address_space *vm,

drivers/gpu/drm/i915/i915_gem_gtt.h

···
 
 	bool closed;
 
+	struct mutex mutex; /* protects vma and our lists */
+
 	struct i915_page_dma scratch_page;
 	struct i915_page_table *scratch_pt;
 	struct i915_page_directory *scratch_pd;

drivers/gpu/drm/i915/i915_gem_shrinker.c

···
  */
 
 #include <linux/oom.h>
+#include <linux/sched/mm.h>
 #include <linux/shmem_fs.h>
 #include <linux/slab.h>
 #include <linux/swap.h>
···
 	WARN_ON(unregister_vmap_purge_notifier(&i915->mm.vmap_notifier));
 	WARN_ON(unregister_oom_notifier(&i915->mm.oom_notifier));
 	unregister_shrinker(&i915->mm.shrinker);
+}
+
+void i915_gem_shrinker_taints_mutex(struct mutex *mutex)
+{
+	if (!IS_ENABLED(CONFIG_LOCKDEP))
+		return;
+
+	fs_reclaim_acquire(GFP_KERNEL);
+	mutex_lock(mutex);
+	mutex_unlock(mutex);
+	fs_reclaim_release(GFP_KERNEL);
 }