vdev_raidz_io_start: Ignore empty writes

The below excerpt of a backtrace is from a ztest failure when running
ZoL's ztest.

/openzfs#453 0x00007f03c8060b35 in vdev_queue_io_to_issue (vq=vq@entry=0x99f8a8) at ../../module/zfs/vdev_queue.c:706
/openzfs#454 0x00007f03c806106e in vdev_queue_io (zio=zio@entry=0x7f0350003de0) at ../../module/zfs/vdev_queue.c:747
/openzfs#455 0x00007f03c80818c1 in zio_vdev_io_start (zio=0x7f0350003de0) at ../../module/zfs/zio.c:2659
/openzfs#456 0x00007f03c807f243 in __zio_execute (zio=0x7f0350003de0) at ../../module/zfs/zio.c:1399
/openzfs#457 zio_nowait (zio=0x7f0350003de0) at ../../module/zfs/zio.c:1456
/openzfs#458 0x00007f03c805f71b in vdev_mirror_io_start (zio=0x7f0350003a10) at ../../module/zfs/vdev_mirror.c:374
/openzfs#459 0x00007f03c807f243 in __zio_execute (zio=0x7f0350003a10) at ../../module/zfs/zio.c:1399
/openzfs#460 zio_nowait (zio=0x7f0350003a10) at ../../module/zfs/zio.c:1456
/openzfs#461 0x00007f03c806464c in vdev_raidz_io_start (zio=0x7f0350003380) at ../../module/zfs/vdev_raidz.c:1607
/openzfs#462 0x00007f03c807f243 in __zio_execute (zio=0x7f0350003380) at ../../module/zfs/zio.c:1399
/openzfs#463 zio_nowait (zio=0x7f0350003380) at ../../module/zfs/zio.c:1456
/openzfs#464 0x00007f03c805f71b in vdev_mirror_io_start (zio=0x7f0350002fb0) at ../../module/zfs/vdev_mirror.c:374
/openzfs#465 0x00007f03c807f243 in __zio_execute (zio=0x7f0350002fb0) at ../../module/zfs/zio.c:1399
/openzfs#466 zio_nowait (zio=0x7f0350002fb0) at ../../module/zfs/zio.c:1456
/openzfs#467 0x00007f03c805ed43 in vdev_mirror_io_done (zio=0x7f033957ebf0) at ../../module/zfs/vdev_mirror.c:499
/openzfs#468 0x00007f03c807a0c0 in zio_vdev_io_done (zio=0x7f033957ebf0) at ../../module/zfs/zio.c:2707
/openzfs#469 0x00007f03c808285b in __zio_execute (zio=0x7f033957ebf0) at ../../module/zfs/zio.c:1399
/openzfs#470 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f0390001330, pio=0x7f033957ebf0) at ../../module/zfs/zio.c:547
/openzfs#471 zio_done (zio=0x7f0390001330) at ../../module/zfs/zio.c:3278
/openzfs#472 0x00007f03c808285b in __zio_execute (zio=0x7f0390001330) at ../../module/zfs/zio.c:1399
/openzfs#473 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f03b4013a00, pio=0x7f0390001330) at ../../module/zfs/zio.c:547
/openzfs#474 zio_done (zio=0x7f03b4013a00) at ../../module/zfs/zio.c:3278
/openzfs#475 0x00007f03c808285b in __zio_execute (zio=0x7f03b4013a00) at ../../module/zfs/zio.c:1399
/openzfs#476 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f03b4014210, pio=0x7f03b4013a00) at ../../module/zfs/zio.c:547
/openzfs#477 zio_done (zio=0x7f03b4014210) at ../../module/zfs/zio.c:3278
/openzfs#478 0x00007f03c808285b in __zio_execute (zio=0x7f03b4014210) at ../../module/zfs/zio.c:1399
/openzfs#479 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f03b4014620, pio=0x7f03b4014210) at ../../module/zfs/zio.c:547
/openzfs#480 zio_done (zio=0x7f03b4014620) at ../../module/zfs/zio.c:3278
/openzfs#481 0x00007f03c807a6d3 in __zio_execute (zio=0x7f03b4014620) at ../../module/zfs/zio.c:1399
/openzfs#482 zio_execute (zio=zio@entry=0x7f03b4014620) at ../../module/zfs/zio.c:1337
/openzfs#483 0x00007f03c8060b35 in vdev_queue_io_to_issue (vq=vq@entry=0x99f8a8) at ../../module/zfs/vdev_queue.c:706
/openzfs#484 0x00007f03c806106e in vdev_queue_io (zio=zio@entry=0x7f0350002be0) at ../../module/zfs/vdev_queue.c:747
/openzfs#485 0x00007f03c80818c1 in zio_vdev_io_start (zio=0x7f0350002be0) at ../../module/zfs/zio.c:2659
/openzfs#486 0x00007f03c807f243 in __zio_execute (zio=0x7f0350002be0) at ../../module/zfs/zio.c:1399
/openzfs#487 zio_nowait (zio=0x7f0350002be0) at ../../module/zfs/zio.c:1456
/openzfs#488 0x00007f03c805f71b in vdev_mirror_io_start (zio=0x7f0350002810) at ../../module/zfs/vdev_mirror.c:374
/openzfs#489 0x00007f03c807f243 in __zio_execute (zio=0x7f0350002810) at ../../module/zfs/zio.c:1399
/openzfs#490 zio_nowait (zio=0x7f0350002810) at ../../module/zfs/zio.c:1456
/openzfs#491 0x00007f03c8064593 in vdev_raidz_io_start (zio=0x7f0350001270) at ../../module/zfs/vdev_raidz.c:1591
/openzfs#492 0x00007f03c807f243 in __zio_execute (zio=0x7f0350001270) at ../../module/zfs/zio.c:1399
/openzfs#493 zio_nowait (zio=0x7f0350001270) at ../../module/zfs/zio.c:1456
/openzfs#494 0x00007f03c805f71b in vdev_mirror_io_start (zio=0x7f0350001e60) at ../../module/zfs/vdev_mirror.c:374
/openzfs#495 0x00007f03c807f243 in __zio_execute (zio=0x7f0350001e60) at ../../module/zfs/zio.c:1399
/openzfs#496 zio_nowait (zio=0x7f0350001e60) at ../../module/zfs/zio.c:1456
/openzfs#497 0x00007f03c805ed43 in vdev_mirror_io_done (zio=0x7f033a0c39c0) at ../../module/zfs/vdev_mirror.c:499
/openzfs#498 0x00007f03c807a0c0 in zio_vdev_io_done (zio=0x7f033a0c39c0) at ../../module/zfs/zio.c:2707
/openzfs#499 0x00007f03c808285b in __zio_execute (zio=0x7f033a0c39c0) at ../../module/zfs/zio.c:1399
/openzfs#500 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f03a8003c00, pio=0x7f033a0c39c0) at ../../module/zfs/zio.c:547
/openzfs#501 zio_done (zio=0x7f03a8003c00) at ../../module/zfs/zio.c:3278
/openzfs#502 0x00007f03c808285b in __zio_execute (zio=0x7f03a8003c00) at ../../module/zfs/zio.c:1399
/openzfs#503 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f038800c400, pio=0x7f03a8003c00) at ../../module/zfs/zio.c:547
/openzfs#504 zio_done (zio=0x7f038800c400) at ../../module/zfs/zio.c:3278
/openzfs#505 0x00007f03c808285b in __zio_execute (zio=0x7f038800c400) at ../../module/zfs/zio.c:1399
/openzfs#506 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f038800da00, pio=0x7f038800c400) at ../../module/zfs/zio.c:547
/openzfs#507 zio_done (zio=0x7f038800da00) at ../../module/zfs/zio.c:3278
/openzfs#508 0x00007f03c808285b in __zio_execute (zio=0x7f038800da00) at ../../module/zfs/zio.c:1399
/openzfs#509 zio_notify_parent (wait=ZIO_WAIT_DONE, zio=0x7f038800fd80, pio=0x7f038800da00) at ../../module/zfs/zio.c:547
/openzfs#510 zio_done (zio=0x7f038800fd80) at ../../module/zfs/zio.c:3278
/openzfs#511 0x00007f03c807a6d3 in __zio_execute (zio=0x7f038800fd80) at ../../module/zfs/zio.c:1399
/openzfs#512 zio_execute (zio=zio@entry=0x7f038800fd80) at ../../module/zfs/zio.c:1337
/openzfs#513 0x00007f03c8060b35 in vdev_queue_io_to_issue (vq=vq@entry=0x99f8a8) at ../../module/zfs/vdev_queue.c:706
/openzfs#514 0x00007f03c806119d in vdev_queue_io_done (zio=zio@entry=0x7f03a0010950) at ../../module/zfs/vdev_queue.c:775
/openzfs#515 0x00007f03c807a0e8 in zio_vdev_io_done (zio=0x7f03a0010950) at ../../module/zfs/zio.c:2686
/openzfs#516 0x00007f03c807a6d3 in __zio_execute (zio=0x7f03a0010950) at ../../module/zfs/zio.c:1399
/openzfs#517 zio_execute (zio=0x7f03a0010950) at ../../module/zfs/zio.c:1337
/openzfs#518 0x00007f03c7fcd0c4 in taskq_thread (arg=0x966d50) at ../../lib/libzpool/taskq.c:215
/openzfs#519 0x00007f03c7fc7937 in zk_thread_helper (arg=0x967e90) at ../../lib/libzpool/kernel.c:135
/openzfs#520 0x00007f03c78890a3 in start_thread (arg=0x7f03c2703700) at pthread_create.c:309
/openzfs#521 0x00007f03c75c50fd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

The backtrace was an infinite loop of `vdev_queue_io_to_issue()` invoking
`zio_execute()` until it overran the stack. vdev_queue_io_to_issue() will ony
invoke `zio_execute()` on raidz vdevs when aggregation I/Os are generated to
improve aggregation continuity. These I/Os do not trigger any writes. However,
it appears that they can be generated in such a way that they recurse
infinitely upon return to `vdev_queue_io_to_issue()`. As a consequence, we see
the number of parents by 1 each time the recursion returns to
`vdev_raidz_io_start()`.

Signed-off-by: Richard Yao <[email protected]>

module/zfs/vdev_raidz.c

@@ -1577,6 +1577,13 @@ vdev_raidz_io_start(zio_t *zio)
 	raidz_col_t *rc;
 	int c, i;
 
+	/*
+	 * Optional I/Os that return to us have served their purpose for
+	 * improving aggregation continuity and can be ignored.
+	 */
+	if (zio->io_type == ZIO_TYPE_WRITE && zio->io_flags & ZIO_FLAG_NODATA)
+		return (ZIO_PIPELINE_CONTINUE);
+
 	rm = vdev_raidz_map_alloc(zio, tvd->vdev_ashift, vd->vdev_children,
 	    vd->vdev_nparity);