drm/i915: use might_lock_nested in get_pages annotation

So strictly speaking the existing annotation is also ok, because we
have a chain of

obj->mm.lock#I915_MM_GET_PAGES -> fs_reclaim -> obj->mm.lock

(the shrinker cannot get at an object while we're in get_pages, hence
this is safe). But it's confusing, so try to take the right subclass
of the lock.

This does a bit reduce our lockdep based checking, but then it's also
less fragile, in case we ever change the nesting around.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191104173720.2696-3-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/gem/i915_gem_object.h

@@ -271,10 +271,27 @@ void __i915_gem_object_set_pages(struct drm_i915_gem_object *obj,
 int ____i915_gem_object_get_pages(struct drm_i915_gem_object *obj);
 int __i915_gem_object_get_pages(struct drm_i915_gem_object *obj);
 
+enum i915_mm_subclass { /* lockdep subclass for obj->mm.lock/struct_mutex */
+	I915_MM_NORMAL = 0,
+	/*
+	 * Only used by struct_mutex, when called "recursively" from
+	 * direct-reclaim-esque. Safe because there is only every one
+	 * struct_mutex in the entire system.
+	 */
+	I915_MM_SHRINKER = 1,
+	/*
+	 * Used for obj->mm.lock when allocating pages. Safe because the object
+	 * isn't yet on any LRU, and therefore the shrinker can't deadlock on
+	 * it. As soon as the object has pages, obj->mm.lock nests within
+	 * fs_reclaim.
+	 */
+	I915_MM_GET_PAGES = 1,
+};
+
 static inline int __must_check
 i915_gem_object_pin_pages(struct drm_i915_gem_object *obj)
 {
-	might_lock(&obj->mm.lock);
+	might_lock_nested(&obj->mm.lock, I915_MM_GET_PAGES);
 
 	if (atomic_inc_not_zero(&obj->mm.pages_pin_count))
 		return 0;
@@ -317,23 +334,6 @@ i915_gem_object_unpin_pages(struct drm_i915_gem_object *obj)
 	__i915_gem_object_unpin_pages(obj);
 }
 
-enum i915_mm_subclass { /* lockdep subclass for obj->mm.lock/struct_mutex */
-	I915_MM_NORMAL = 0,
-	/*
-	 * Only used by struct_mutex, when called "recursively" from
-	 * direct-reclaim-esque. Safe because there is only every one
-	 * struct_mutex in the entire system.
-	 */
-	I915_MM_SHRINKER = 1,
-	/*
-	 * Used for obj->mm.lock when allocating pages. Safe because the object
-	 * isn't yet on any LRU, and therefore the shrinker can't deadlock on
-	 * it. As soon as the object has pages, obj->mm.lock nests within
-	 * fs_reclaim.
-	 */
-	I915_MM_GET_PAGES = 1,
-};
-
 int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj);
 void i915_gem_object_truncate(struct drm_i915_gem_object *obj);
 void i915_gem_object_writeback(struct drm_i915_gem_object *obj);