i915/selftest/igt_mmap: let mmap tests run in kthread

When the driver is loaded on the system with numa nodes it might be run in
a kthread, which makes it impossible to use current->mm in the selftest.
This patch allows the selftest to use current->mm by using active_mm.

Signed-off-by: Mikolaj Wasiak <mikolaj.wasiak@intel.com>
Reviewed-by: Eugene Kobyak <eugene.kobyak@intel.com>
Reviewed-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2w6pt2hnemndwmanwhyn3keexa6vtha7rmo6rqoerkmyxhbrh2@ls7lndjpia6z

Author: mwasiakint

drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c

@@ -1837,6 +1837,8 @@ static int igt_mmap_revoke(void *arg)
 
 int i915_gem_mman_live_selftests(struct drm_i915_private *i915)
 {
+	int ret;
+	bool unuse_mm = false;
 	static const struct i915_subtest tests[] = {
 		SUBTEST(igt_partial_tiling),
 		SUBTEST(igt_smoke_tiling),
@@ -1848,5 +1850,15 @@ int i915_gem_mman_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(igt_mmap_gpu),
 	};
 
-	return i915_live_subtests(tests, i915);
+	if (!current->mm) {
+		kthread_use_mm(current->active_mm);
+		unuse_mm = true;
+	}
+
+	ret = i915_live_subtests(tests, i915);
+
+	if (unuse_mm)
+		kthread_unuse_mm(current->active_mm);
+
+	return ret;
 }