thp: move khugepaged_mutex out of khugepaged

Currently, hugepaged_mutex is used really complexly and hard to
understand, actually, it is just used to serialize start_khugepaged and
khugepaged for these reasons:

- khugepaged_thread is shared between them
- the thp disable path (echo never > transparent_hugepage/enabled) is
  nonblocking, so we need to protect khugepaged_thread to get a stable
  running state

These can be avoided by:

- use the lock to serialize the thread creation and cancel
- thp disable path can not finised until the thread exits

Then khugepaged_thread is fully controlled by start_khugepaged, khugepaged
will be happy without the lock

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 23 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9833d8ecf38..0931b2b19c5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -139,9 +139,6 @@ static int start_khugepaged(void)
 {
         int err = 0;
         if (khugepaged_enabled()) {
-                int wakeup;
-
-                mutex_lock(&khugepaged_mutex);
                 if (!khugepaged_thread)
                         khugepaged_thread = kthread_run(khugepaged, NULL,
                                                         "khugepaged");
@@ -151,15 +148,17 @@ static int start_khugepaged(void)
                         err = PTR_ERR(khugepaged_thread);
                         khugepaged_thread = NULL;
                 }
-                wakeup = !list_empty(&khugepaged_scan.mm_head);
-                mutex_unlock(&khugepaged_mutex);
-                if (wakeup)
+
+                if (!list_empty(&khugepaged_scan.mm_head))
                         wake_up_interruptible(&khugepaged_wait);
 
                 set_recommended_min_free_kbytes();
-        } else
+        } else if (khugepaged_thread) {
                 /* wakeup to exit */
                 wake_up_interruptible(&khugepaged_wait);
+                kthread_stop(khugepaged_thread);
+                khugepaged_thread = NULL;
+        }
 
         return err;
 }
@@ -221,7 +220,12 @@ static ssize_t enabled_store(struct kobject *kobj,
                                 TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG);
 
         if (ret > 0) {
-                int err = start_khugepaged();
+                int err;
+
+                mutex_lock(&khugepaged_mutex);
+                err = start_khugepaged();
+                mutex_unlock(&khugepaged_mutex);
+
                 if (err)
                         ret = err;
         }
@@ -2329,20 +2333,10 @@ static int khugepaged(void *none)
         set_freezable();
         set_user_nice(current, 19);
 
-        /* serialize with start_khugepaged() */
-        mutex_lock(&khugepaged_mutex);
-
-        for (;;) {
-                mutex_unlock(&khugepaged_mutex);
+        while (!kthread_should_stop()) {
                 VM_BUG_ON(khugepaged_thread != current);
                 khugepaged_loop();
                 VM_BUG_ON(khugepaged_thread != current);
-
-                mutex_lock(&khugepaged_mutex);
-                if (!khugepaged_enabled())
-                        break;
-                if (unlikely(kthread_should_stop()))
-                        break;
         }
 
         spin_lock(&khugepaged_mm_lock);
@@ -2351,10 +2345,6 @@ static int khugepaged(void *none)
         if (mm_slot)
                 collect_mm_slot(mm_slot);
         spin_unlock(&khugepaged_mm_lock);
-
-        khugepaged_thread = NULL;
-        mutex_unlock(&khugepaged_mutex);
-
         return 0;
 }