[PATCH] mm: Restore mmput_async

[Sherry Yang <sherryy@android.com>]

Restore asynchronous mmput, allowing mmput_async to be called
from an atomic context in Android binder shrinker callback.

mmput_async was initially introduced in ec8d7c14e
("mm, oom_reaper: do not mmput synchronously from the
oom reaper context"), and was removed in 212925802
("mm: oom: let oom_reap_task and exit_mmap run concurrently")

Signed-off-by: Sherry Yang <sherryy@android.com>
---
 include/linux/sched/mm.h |  6 ++++++
 kernel/fork.c            | 18 ++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3a19c253bdb1..ae53e413fb13 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -84,6 +84,12 @@ static inline bool mmget_not_zero(struct mm_struct *mm)
 
 /* mmput gets rid of the mappings and all user-space */
 extern void mmput(struct mm_struct *);
+#ifdef CONFIG_MMU
+/* same as above but performs the slow path from the async context. Can
+ * be called from the atomic context as well
+ */
+void mmput_async(struct mm_struct *);
+#endif
 
 /* Grab a reference to a task's mm, if it is not already going away */
 extern struct mm_struct *get_task_mm(struct task_struct *task);
diff --git a/kernel/fork.c b/kernel/fork.c
index 10646182440f..e702cb9ffbd8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -946,6 +946,24 @@ void mmput(struct mm_struct *mm)
 }
 EXPORT_SYMBOL_GPL(mmput);
 
+#ifdef CONFIG_MMU
+static void mmput_async_fn(struct work_struct *work)
+{
+	struct mm_struct *mm = container_of(work, struct mm_struct,
+					    async_put_work);
+
+	__mmput(mm);
+}
+
+void mmput_async(struct mm_struct *mm)
+{
+	if (atomic_dec_and_test(&mm->mm_users)) {
+		INIT_WORK(&mm->async_put_work, mmput_async_fn);
+		schedule_work(&mm->async_put_work);
+	}
+}
+#endif
+
 /**
  * set_mm_exe_file - change a reference to the mm's executable file
  *
-- 
2.11.0 (Apple Git-81)