Fix a race condition in dsl_dataset_sync() when activating features

The zio returned from arc_write() in dmu_objset_sync() uses
zio_nowait(). However we may reach the end of dsl_dataset_sync()
which checks if we need to activate features in the filesystem
without knowing if that zio has even run through the ZIO pipeline yet.
In that case we will flag features to be activated in
dsl_dataset_block_born() but dsl_dataset_sync() has already
completed its run and those features will not actually be activated.
Mitigate this by moving the feature activation code in the callback of
arc_write(), dmu_objset_write_done(). Also add new ASSERTs in
dsl_scan_visitbp() checking if a block contradicts any filesystem
flags.

Signed-off-by: George Amanakis <gamanakis@gmail.com>

include/sys/dsl_dataset.h

@@ -372,6 +372,7 @@ int dsl_dataset_rename_snapshot(const char *fsname,
     const char *oldsnapname, const char *newsnapname, boolean_t recursive);
 int dsl_dataset_snapshot_tmp(const char *fsname, const char *snapname,
     minor_t cleanup_minor, const char *htag);
+boolean_t zfeature_active(spa_feature_t f, void *arg);
 
 blkptr_t *dsl_dataset_get_blkptr(dsl_dataset_t *ds);
 

module/zfs/dmu_objset.c

@@ -1613,6 +1613,22 @@ dmu_objset_write_done(zio_t *zio, arc_buf_t *abuf, void *arg)
 
 		(void) dsl_dataset_block_kill(ds, bp_orig, tx, B_TRUE);
 		dsl_dataset_block_born(ds, bp, tx);
+
+		if (ds != NULL) {
+			for (spa_feature_t f = 0; f < SPA_FEATURES; f++) {
+				if (zfeature_active(f,
+				    ds->ds_feature_activation[f])) {
+					if (zfeature_active(f,
+					    ds->ds_feature[f]))
+						continue;
+					dsl_dataset_activate_feature(
+					    ds->ds_object, f,
+					    ds->ds_feature_activation[f], tx);
+					ds->ds_feature[f] =
+					    ds->ds_feature_activation[f];
+				}
+			}
+		}
 	}
 	kmem_free(bp, sizeof (*bp));
 }

module/zfs/dmu_send.c

@@ -493,6 +493,7 @@ dmu_dump_write(dmu_send_cookie_t *dscp, dmu_object_type_t type, uint64_t object,
 	    (bp != NULL ? BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF &&
 	    io_compressed : lsize != psize);
 	if (raw || compressed) {
+		ASSERT(bp != NULL);
 		ASSERT(raw || dscp->dsc_featureflags &
 		    DMU_BACKUP_FEATURE_COMPRESSED);
 		ASSERT(!BP_IS_EMBEDDED(bp));

module/zfs/dsl_dataset.c

@@ -1039,7 +1039,7 @@ dsl_dataset_has_owner(dsl_dataset_t *ds)
 	return (rv);
 }
 
-static boolean_t
+boolean_t
 zfeature_active(spa_feature_t f, void *arg)
 {
 	switch (spa_feature_table[f].fi_type) {
@@ -2118,16 +2118,6 @@ dsl_dataset_sync(dsl_dataset_t *ds, zio_t *zio, dmu_tx_t *tx)
 	}
 
 	dmu_objset_sync(ds->ds_objset, zio, tx);
-
-	for (spa_feature_t f = 0; f < SPA_FEATURES; f++) {
-		if (zfeature_active(f, ds->ds_feature_activation[f])) {
-			if (zfeature_active(f, ds->ds_feature[f]))
-				continue;
-			dsl_dataset_activate_feature(ds->ds_object, f,
-			    ds->ds_feature_activation[f], tx);
-			ds->ds_feature[f] = ds->ds_feature_activation[f];
-		}
-	}
 }
 
 /*

module/zfs/dsl_scan.c

@@ -2024,6 +2024,26 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
 		return;
 	}
 
+	/*
+	 * Check if this block contradicts any filesystem flags.
+	 */
+	spa_feature_t f = SPA_FEATURE_LARGE_BLOCKS;
+	if (BP_GET_LSIZE(bp) > SPA_OLD_MAXBLOCKSIZE)
+		ASSERT3B(dsl_dataset_feature_is_active(ds, f), ==, B_TRUE);
+
+	f = zio_checksum_to_feature(BP_GET_CHECKSUM(bp));
+	if (f != SPA_FEATURE_NONE)
+		ASSERT3B(dsl_dataset_feature_is_active(ds, f), ==, B_TRUE);
+
+	f = zio_compress_to_feature(BP_GET_COMPRESS(bp));
+	if (f != SPA_FEATURE_NONE)
+		ASSERT3B(dsl_dataset_feature_is_active(ds, f), ==, B_TRUE);
+
+	if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg) {
+		scn->scn_lt_min_this_txg++;
+		return;
+	}
+
 	if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg) {
 		scn->scn_lt_min_this_txg++;
 		return;