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.

The illumos bug for this is illumos#15939.

Signed-off-by: Jason King <[email protected]>

Author: jasonbking

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);