[PATCH] sched/core: Don't use dying mm as active_mm for kernel threads

From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Phil Auld <pauld@redhat.com>, Waiman Long <longman@redhat.com>
Subject: [PATCH] sched/core: Don't use dying mm as active_mm for kernel threads
Date: Fri, 26 Jul 2019 19:45:41 -0400	[thread overview]
Message-ID: <20190726234541.3771-1-longman@redhat.com> (raw)

It was found that a dying mm_struct where the owning task has exited can
stay on as active_mm of kernel threads as long as no other user tasks
run on those CPUs that use it as active_mm. This prolongs the life time
of dying mm holding up memory and other resources that cannot be freed.

Fix that by forcing the kernel threads to use init_mm as the active_mm
if the previous active_mm is dying.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/core.c | 13 +++++++++++--
 mm/init-mm.c        |  2 ++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2b037f195473..ca348e1f5a1e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3233,13 +3233,22 @@ context_switch(struct rq *rq, struct task_struct *prev,
 	 * Both of these contain the full memory barrier required by
 	 * membarrier after storing to rq->curr, before returning to
 	 * user-space.
+	 *
+	 * If mm is NULL and oldmm is dying (!owner), we switch to
+	 * init_mm instead to make sure that oldmm can be freed ASAP.
 	 */
-	if (!mm) {
+	if (!mm && oldmm->owner) {
 		next->active_mm = oldmm;
 		mmgrab(oldmm);
 		enter_lazy_tlb(oldmm, next);
-	} else
+	} else {
+		if (!mm) {
+			mm = &init_mm;
+			next->active_mm = mm;
+			mmgrab(mm);
+		}
 		switch_mm_irqs_off(oldmm, mm, next);
+	}
 
 	if (!prev->mm) {
 		prev->active_mm = NULL;
diff --git a/mm/init-mm.c b/mm/init-mm.c
index a787a319211e..5bfc6bc333ca 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -5,6 +5,7 @@
 #include <linux/spinlock.h>
 #include <linux/list.h>
 #include <linux/cpumask.h>
+#include <linux/sched/task.h>
 
 #include <linux/atomic.h>
 #include <linux/user_namespace.h>
@@ -36,5 +37,6 @@ struct mm_struct init_mm = {
 	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
 	.user_ns	= &init_user_ns,
 	.cpu_bitmap	= { [BITS_TO_LONGS(NR_CPUS)] = 0},
+	.owner		= &init_task,
 	INIT_MM_CONTEXT(init_mm)
 };
-- 
2.18.1

