Remove db_state DB_NOFILL checks from syncing context

Syncing context should not depend on current state of dbuf, which
could already change several times in later transaction groups,
but rely solely on dirty record for the transaction group being
synced. Some of the checks seem already impossible, while instead
of others I think we should better check for absence of data in
the specific dirty record rather than DB_NOFILL.

Reviewed-by: Robert Evans <evansr@google.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#16057

module/zfs/dbuf.c

@@ -4597,11 +4597,10 @@ dbuf_sync_leaf(dbuf_dirty_record_t *dr, dmu_tx_t *tx)
 	if (os->os_encrypted && dn->dn_object == DMU_META_DNODE_OBJECT)
 		dbuf_prepare_encrypted_dnode_leaf(dr);
 
-	if (db->db_state != DB_NOFILL &&
+	if (*datap != NULL && *datap == db->db_buf &&
 	    dn->dn_object != DMU_META_DNODE_OBJECT &&
 	    zfs_refcount_count(&db->db_holds) > 1 &&
-	    dr->dt.dl.dr_override_state != DR_OVERRIDDEN &&
-	    *datap == db->db_buf) {
+	    dr->dt.dl.dr_override_state != DR_OVERRIDDEN) {
 		/*
 		 * If this buffer is currently "in use" (i.e., there
 		 * are active holds and db_data still references it),
@@ -4890,11 +4889,9 @@ dbuf_write_done(zio_t *zio, arc_buf_t *buf, void *vdb)
 	if (db->db_level == 0) {
 		ASSERT(db->db_blkid != DMU_BONUS_BLKID);
 		ASSERT(dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN);
-		if (db->db_state != DB_NOFILL) {
-			if (dr->dt.dl.dr_data != NULL &&
-			    dr->dt.dl.dr_data != db->db_buf) {
-				arc_buf_destroy(dr->dt.dl.dr_data, db);
-			}
+		if (dr->dt.dl.dr_data != NULL &&
+		    dr->dt.dl.dr_data != db->db_buf) {
+			arc_buf_destroy(dr->dt.dl.dr_data, db);
 		}
 	} else {
 		ASSERT(list_head(&dr->dt.di.dr_children) == NULL);
@@ -5097,21 +5094,18 @@ dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx)
 
 	os = dn->dn_objset;
 
-	if (db->db_state != DB_NOFILL) {
-		if (db->db_level > 0 || dn->dn_type == DMU_OT_DNODE) {
-			/*
-			 * Private object buffers are released here rather
-			 * than in dbuf_dirty() since they are only modified
-			 * in the syncing context and we don't want the
-			 * overhead of making multiple copies of the data.
-			 */
-			if (BP_IS_HOLE(db->db_blkptr)) {
-				arc_buf_thaw(data);
-			} else {
-				dbuf_release_bp(db);
-			}
-			dbuf_remap(dn, db, tx);
-		}
+	if (db->db_level > 0 || dn->dn_type == DMU_OT_DNODE) {
+		/*
+		 * Private object buffers are released here rather than in
+		 * dbuf_dirty() since they are only modified in the syncing
+		 * context and we don't want the overhead of making multiple
+		 * copies of the data.
+		 */
+		if (BP_IS_HOLE(db->db_blkptr))
+			arc_buf_thaw(data);
+		else
+			dbuf_release_bp(db);
+		dbuf_remap(dn, db, tx);
 	}
 
 	if (parent != dn->dn_dbuf) {
@@ -5147,7 +5141,7 @@ dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx)
 
 	if (db->db_blkid == DMU_SPILL_BLKID)
 		wp_flag = WP_SPILL;
-	wp_flag |= (db->db_state == DB_NOFILL) ? WP_NOFILL : 0;
+	wp_flag |= (data == NULL) ? WP_NOFILL : 0;
 
 	dmu_write_policy(os, dn, db->db_level, wp_flag, &zp);
 
@@ -5179,7 +5173,7 @@ dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx)
 		    dr->dt.dl.dr_copies, dr->dt.dl.dr_nopwrite,
 		    dr->dt.dl.dr_brtwrite);
 		mutex_exit(&db->db_mtx);
-	} else if (db->db_state == DB_NOFILL) {
+	} else if (data == NULL) {
 		ASSERT(zp.zp_checksum == ZIO_CHECKSUM_OFF ||
 		    zp.zp_checksum == ZIO_CHECKSUM_NOPARITY);
 		dr->dr_zio = zio_write(pio, os->os_spa, txg,