Zpool can start allocating from metaslab before TRIMs have completed

When doing a manual TRIM on a zpool, the metaslab being TRIMmed is
potentially re-enabled before all queued TRIM zios for that metaslab
have completed. Since TRIM zios have the lowest priority, it is 
possible to get into a situation where allocations occur from the 
just re-enabled metaslab and cut ahead of queued TRIMs to the same 
metaslab.  If the ranges overlap, this will cause corruption.

We were able to trigger this pretty consistently with a small single 
top-level vdev zpool (i.e. small number of metaslabs) with heavy 
parallel write activity while performing a manual TRIM against a 
somewhat 'slow' device (so TRIMs took a bit of time to complete). 
With the patch, we've not been able to recreate it since. It was on 
illumos, but inspection of the OpenZFS trim code looks like the 
relevant pieces are largely unchanged and so it appears it would be 
vulnerable to the same issue.

Reviewed-by: Igor Kozhukhov <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Jason King <[email protected]>
Illumos-issue: https://www.illumos.org/issues/15939
Closes #15395

module/zfs/vdev_trim.c

@@ -23,6 +23,7 @@
  * Copyright (c) 2016 by Delphix. All rights reserved.
  * Copyright (c) 2019 by Lawrence Livermore National Security, LLC.
  * Copyright (c) 2021 Hewlett Packard Enterprise Development LP
+ * Copyright 2023 RackTop Systems, Inc.
  */
 
 #include <sys/spa.h>
@@ -591,6 +592,7 @@ vdev_trim_ranges(trim_args_t *ta)
 	uint64_t extent_bytes_max = ta->trim_extent_bytes_max;
 	uint64_t extent_bytes_min = ta->trim_extent_bytes_min;
 	spa_t *spa = vd->vdev_spa;
+	int error = 0;
 
 	ta->trim_start_time = gethrtime();
 	ta->trim_bytes_done = 0;
@@ -610,19 +612,32 @@ vdev_trim_ranges(trim_args_t *ta)
 		uint64_t writes_required = ((size - 1) / extent_bytes_max) + 1;
 
 		for (uint64_t w = 0; w < writes_required; w++) {
-			int error;
-
 			error = vdev_trim_range(ta, VDEV_LABEL_START_SIZE +
 			    rs_get_start(rs, ta->trim_tree) +
 			    (w *extent_bytes_max), MIN(size -
 			    (w * extent_bytes_max), extent_bytes_max));
 			if (error != 0) {
-				return (error);
+				goto done;
 			}
 		}
 	}
 
-	return (0);
+done:
+	/*
+	 * Make sure all TRIMs for this metaslab have completed before
+	 * returning. TRIM zios have lower priority over regular or syncing
+	 * zios, so all TRIM zios for this metaslab must complete before the
+	 * metaslab is re-enabled. Otherwise it's possible write zios to
+	 * this metaslab could cut ahead of still queued TRIM zios for this
+	 * metaslab causing corruption if the ranges overlap.
+	 */
+	mutex_enter(&vd->vdev_trim_io_lock);
+	while (vd->vdev_trim_inflight[0] > 0) {
+		cv_wait(&vd->vdev_trim_io_cv, &vd->vdev_trim_io_lock);
+	}
+	mutex_exit(&vd->vdev_trim_io_lock);
+
+	return (error);
 }
 
 static void
@@ -941,11 +956,6 @@ vdev_trim_thread(void *arg)
 	}
 
 	spa_config_exit(spa, SCL_CONFIG, FTAG);
-	mutex_enter(&vd->vdev_trim_io_lock);
-	while (vd->vdev_trim_inflight[0] > 0) {
-		cv_wait(&vd->vdev_trim_io_cv, &vd->vdev_trim_io_lock);
-	}
-	mutex_exit(&vd->vdev_trim_io_lock);
 
 	range_tree_destroy(ta.trim_tree);