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