Improve TXG handling in the TRIM module.

This patch adds some improvements to the way the trim module considers
TXGs:

 - Free ZIOs are registered with the TXG from the ZIO itself, not the
   current SPA syncing TXG (which may be out of date);
 - L2ARC are registered with a zero TXG number, as L2ARC has no concept
   of TXGs;
 - The TXG limit for issuing TRIMs is now computed from the last synced
   TXG, not the currently syncing TXG. Indeed, under extremely unlikely
   race conditions, there is a risk we could trim blocks which have been
   freed in a TXG that has not finished syncing, resulting in potential
   data corruption in case of a crash.

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
Obtained from:	https://github.com/dechamps/zfs/commit/5b46ad40d9081d75505d6f3bf04ac652445df366
MFC after:	2 weeks

1 files changed, 4 insertions, 5 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/trim_map.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/trim_map.c
index bd24d4a..3a72936 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/trim_map.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/trim_map.c
@@ -272,7 +272,7 @@ trim_map_free_locked(trim_map_t *tm, uint64_t start, uint64_t end, uint64_t txg)
 }
 
 void
-trim_map_free(vdev_t *vd, uint64_t offset, uint64_t size)
+trim_map_free(vdev_t *vd, uint64_t offset, uint64_t size, uint64_t txg)
 {
 	trim_map_t *tm = vd->vdev_trimmap;
 
@@ -280,8 +280,7 @@ trim_map_free(vdev_t *vd, uint64_t offset, uint64_t size)
 		return;
 
 	mutex_enter(&tm->tm_lock);
-	trim_map_free_locked(tm, offset, TRIM_ZIO_END(vd, offset, size),
-	    vd->vdev_spa->spa_syncing_txg);
+	trim_map_free_locked(tm, offset, TRIM_ZIO_END(vd, offset, size), txg);
 	mutex_exit(&tm->tm_lock);
 }
 
@@ -387,7 +386,7 @@ trim_map_vdev_commit(spa_t *spa, zio_t *zio, vdev_t *vd)
 	if (tm == NULL)
 		return;
 
-	txglimit = MIN(spa->spa_syncing_txg, spa_freeze_txg(spa)) -
+	txglimit = MIN(spa_last_synced_txg(spa), spa_freeze_txg(spa)) -
 	    trim_txg_limit;
 
 	mutex_enter(&tm->tm_lock);
@@ -444,7 +443,7 @@ trim_map_commit(spa_t *spa, zio_t *zio, vdev_t *vd)
 {
 	int c;
 
-	if (vd == NULL || spa->spa_syncing_txg <= trim_txg_limit)
+	if (vd == NULL || spa_last_synced_txg(spa) <= trim_txg_limit)
 		return;
 
 	if (vd->vdev_ops->vdev_op_leaf) {