[PATCH] mm: fix pgdat->kswap accessed concurrently

[Kefeng Wang]

The pgdat->kswap could be accessed concurrently by kswapd_run() and
kcompactd(), it don't be protected by any lock, which leads to the
following null-ptr-deref,

  vmscan: Failed to start kswapd on node 0
  ...
  BUG: KASAN: null-ptr-deref in kcompactd+0x440/0x504
  Read of size 8 at addr 0000000000000024 by task kcompactd0/37

  CPU: 0 PID: 37 Comm: kcompactd0 Kdump: loaded Tainted: G           OE     5.10.60 #1
  Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
  Call trace:
   dump_backtrace+0x0/0x394
   show_stack+0x34/0x4c
   dump_stack+0x158/0x1e4
   __kasan_report+0x138/0x140
   kasan_report+0x44/0xdc
   __asan_load8+0x94/0xd0
   kcompactd+0x440/0x504
   kthread+0x1a4/0x1f0
   ret_from_fork+0x10/0x18

Fix it by adding READ_ONCE()|WRITE_ONCE().

Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
 mm/compaction.c |  4 +++-
 mm/vmscan.c     | 15 +++++++++------
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 640fa76228dd..aa1cfe47f046 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1983,7 +1983,9 @@ static inline bool is_via_compact_memory(int order)
 
 static bool kswapd_is_running(pg_data_t *pgdat)
 {
-	return pgdat->kswapd && task_is_running(pgdat->kswapd);
+	struct task_struct *t = READ_ONCE(pgdat->kswapd);
+
+	return t && task_is_running(t);
 }
 
 /*
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b2b1431352dc..9abba714249e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4642,16 +4642,19 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
 void kswapd_run(int nid)
 {
 	pg_data_t *pgdat = NODE_DATA(nid);
+	struct task_struct *t;
 
-	if (pgdat->kswapd)
+	if (READ_ONCE(pgdat->kswapd))
 		return;
 
-	pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
-	if (IS_ERR(pgdat->kswapd)) {
+	t = kthread_run(kswapd, pgdat, "kswapd%d", nid);
+	if (IS_ERR(t)) {
 		/* failure at boot is fatal */
 		BUG_ON(system_state < SYSTEM_RUNNING);
 		pr_err("Failed to start kswapd on node %d\n", nid);
-		pgdat->kswapd = NULL;
+		WRITE_ONCE(pgdat->kswapd, NULL);
+	} else {
+		WRITE_ONCE(pgdat->kswapd, t);
 	}
 }
 
@@ -4661,11 +4664,11 @@ void kswapd_run(int nid)
  */
 void kswapd_stop(int nid)
 {
-	struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
+	struct task_struct *kswapd = READ_ONCE(NODE_DATA(nid)->kswapd);
 
 	if (kswapd) {
 		kthread_stop(kswapd);
-		NODE_DATA(nid)->kswapd = NULL;
+		WRITE_ONCE(NODE_DATA(nid)->kswapd, NULL);
 	}
 }
 
-- 
2.35.3