[RFC PATCH 5/6] ptrace: Use mm_ref() for ->exit_mm

[Jann Horn]

We only use ->exit_mm to look up dumpability and the ->user_mm; we don't
need to keep the PGD alive for this.
mmgrab() is also inconvenient here, because it means that we need to use
mmdrop_async() when dropping the reference to the mm from an RCU callback.
Use mm_ref() instead of mmgrab() to make things neater.

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 kernel/exit.c | 2 +-
 kernel/fork.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 97253ef33486..03ba6d13ef1e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -476,7 +476,7 @@ static void exit_mm(void)
 	/* more a memory barrier than a real lock */
 	task_lock(current);
 	current->mm = NULL;
-	mmgrab(mm); /* for current->exit_mm */
+	mm_ref(mm); /* for current->exit_mm */
 	current->exit_mm = mm;
 	mmap_read_unlock(mm);
 	enter_lazy_tlb(mm, current);
diff --git a/kernel/fork.c b/kernel/fork.c
index 59c119b03351..4383bf055b40 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -720,7 +720,7 @@ void free_task(struct task_struct *tsk)
 	if (tsk->flags & PF_KTHREAD)
 		free_kthread_struct(tsk);
 	if (tsk->exit_mm)
-		mmdrop_async(tsk->exit_mm);
+		mm_unref(tsk->exit_mm);
 	free_task_struct(tsk);
 }
 EXPORT_SYMBOL(free_task);
-- 
2.29.0.rc1.297.gfa9743e501-goog