sched/numa: fix memory leak due to the overwritten vma->numab_state

[Problem Description]
When running the hackbench program of LTP, the following memory leak is
reported by kmemleak.

  # /opt/ltp/testcases/bin/hackbench 20 thread 1000
  Running with 20*40 (== 800) tasks.

  # dmesg | grep kmemleak
  ...
  kmemleak: 480 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
  kmemleak: 665 new suspected memory leaks (see /sys/kernel/debug/kmemleak)

  # cat /sys/kernel/debug/kmemleak
  unreferenced object 0xffff888cd8ca2c40 (size 64):
    comm "hackbench", pid 17142, jiffies 4299780315
    hex dump (first 32 bytes):
      ac 74 49 00 01 00 00 00 4c 84 49 00 01 00 00 00  .tI.....L.I.....
      00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    backtrace (crc bff18fd4):
      [<ffffffff81419a89>] __kmalloc_cache_noprof+0x2f9/0x3f0
      [<ffffffff8113f715>] task_numa_work+0x725/0xa00
      [<ffffffff8110f878>] task_work_run+0x58/0x90
      [<ffffffff81ddd9f8>] syscall_exit_to_user_mode+0x1c8/0x1e0
      [<ffffffff81dd78d5>] do_syscall_64+0x85/0x150
      [<ffffffff81e0012b>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
  ...

This issue can be consistently reproduced on three different servers:
  * a 448-core server
  * a 256-core server
  * a 192-core server

[Root Cause]
Since multiple threads are created by the hackbench program (along with
the command argument 'thread'), a shared vma might be accessed by two or
more cores simultaneously. When two or more cores observe that
vma->numab_state is NULL at the same time, vma->numab_state will be
overwritten.

Although current code ensures that only one thread scans the VMAs in a
single 'numa_scan_period', there might be a chance for another thread
to enter in the next 'numa_scan_period' while we have not gotten till
numab_state allocation [1].

Note that the command `/opt/ltp/testcases/bin/hackbench 50 process 1000`
cannot the reproduce the issue. It is verified with 200+ test runs.

[Solution]
Use the cmpxchg atomic operation to ensure that only one thread executes
the vma->numab_state assignment.

[1] https://lore.kernel.org/lkml/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Fixes: ef6a22b ("sched/numa: apply the scan delay to every new vma")
Signed-off-by: Adrian Huang <[email protected]>
Reported-by: Jiwei Sun <[email protected]>
Reviewed-by: Raghavendra K T <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: Ben Segall <[email protected]>
Cc: Dietmar Eggemann <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Vincent Guittot <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Adrian Huang

kernel/sched/fair.c

@@ -3399,10 +3399,16 @@ static void task_numa_work(struct callback_head *work)
 
 		/* Initialise new per-VMA NUMAB state. */
 		if (!vma->numab_state) {
-			vma->numab_state = kzalloc(sizeof(struct vma_numab_state),
-				GFP_KERNEL);
-			if (!vma->numab_state)
+			struct vma_numab_state *ptr;
+
+			ptr = kzalloc(sizeof(*ptr), GFP_KERNEL);
+			if (!ptr)
+				continue;
+
+			if (cmpxchg(&vma->numab_state, NULL, ptr)) {
+				kfree(ptr);
 				continue;
+			}
 
 			vma->numab_state->start_scan_seq = mm->numa_scan_seq;