Make ZIL operations on zvols use _by_dnode routines

This continues what was started in 0eef1bd by
fully converting zvols to avoid unnecessary dnode_hold() calls. This saves a
small amount of CPU time and slightly improves latencies on synchronous
operations on zvols.

Signed-off-by: Richard Yao <richard.yao@prophetstor.com>

module/zfs/dmu.c

@@ -965,6 +965,9 @@ int
 dmu_read_by_dnode(dnode_t *dn, uint64_t offset, uint64_t size, void *buf,
     uint32_t flags)
 {
+	if (size == 0)
+		return (0);
+
 	return (dmu_read_impl(dn, offset, size, buf, flags));
 }
 
@@ -1342,6 +1345,9 @@ dmu_write_uio_dnode(dnode_t *dn, uio_t *uio, uint64_t size, dmu_tx_t *tx)
 	int err = 0;
 	int i;
 
+	if (size == 0)
+		return (0);
+
 	err = dmu_buf_hold_array_by_dnode(dn, uio->uio_loffset, size,
 	    FALSE, FTAG, &numbufs, &dbp, DMU_READ_PREFETCH);
 	if (err)

module/zfs/zvol.c

@@ -111,7 +111,7 @@ struct zvol_state {
 	uint32_t		zv_changed;	/* disk changed */
 	zilog_t			*zv_zilog;	/* ZIL handle */
 	zfs_rlock_t		zv_range_lock;	/* range lock */
-	dmu_buf_t		*zv_dbuf;	/* bonus handle */
+	dmu_buf_t		*zv_dn;		/* bonus dnode */
 	dev_t			zv_dev;		/* device id */
 	struct gendisk		*zv_disk;	/* generic disk */
 	struct request_queue	*zv_queue;	/* request queue */
@@ -647,8 +647,8 @@ zvol_log_write(zvol_state_t *zv, dmu_tx_t *tx, uint64_t offset,
 		itx = zil_itx_create(TX_WRITE, sizeof (*lr) +
 		    (write_state == WR_COPIED ? len : 0));
 		lr = (lr_write_t *)&itx->itx_lr;
-		if (write_state == WR_COPIED && dmu_read(zv->zv_objset,
-		    ZVOL_OBJ, offset, len, lr+1, DMU_READ_NO_PREFETCH) != 0) {
+		if (write_state == WR_COPIED && dmu_read_by_dnode(zv->zv_dn,
+		    offset, len, lr+1, DMU_READ_NO_PREFETCH) != 0) {
 			zil_itx_destroy(itx);
 			itx = zil_itx_create(TX_WRITE, sizeof (*lr));
 			lr = (lr_write_t *)&itx->itx_lr;
@@ -729,7 +729,7 @@ zvol_write(void *arg)
 			dmu_tx_abort(tx);
 			break;
 		}
-		error = dmu_write_uio_dbuf(zv->zv_dbuf, &uio, bytes, tx);
+		error = dmu_write_uio_dnode(zv->zv_dn, &uio, bytes, tx);
 		if (error == 0)
 			zvol_log_write(zv, tx, off, bytes, sync);
 		dmu_tx_commit(tx);
@@ -854,7 +854,7 @@ zvol_read(void *arg)
 		if (bytes > volsize - uio.uio_loffset)
 			bytes = volsize - uio.uio_loffset;
 
-		error = dmu_read_uio_dbuf(zv->zv_dbuf, &uio, bytes);
+		error = dmu_read_uio_dnode(zv->zv_dn, &uio, bytes);
 		if (error) {
 			/* convert checksum errors into IO errors */
 			if (error == ECKSUM)
@@ -978,8 +978,6 @@ static int
 zvol_get_data(void *arg, lr_write_t *lr, char *buf, zio_t *zio)
 {
 	zvol_state_t *zv = arg;
-	objset_t *os = zv->zv_objset;
-	uint64_t object = ZVOL_OBJ;
 	uint64_t offset = lr->lr_offset;
 	uint64_t size = lr->lr_length;
 	blkptr_t *bp = &lr->lr_blkptr;
@@ -1003,12 +1001,12 @@ zvol_get_data(void *arg, lr_write_t *lr, char *buf, zio_t *zio)
 	 * we don't have to write the data twice.
 	 */
 	if (buf != NULL) { /* immediate write */
-		error = dmu_read(os, object, offset, size, buf,
+		error = dmu_read_by_dnode(zv->zv_dn, offset, size, buf,
 		    DMU_READ_NO_PREFETCH);
 	} else {
 		size = zv->zv_volblocksize;
 		offset = P2ALIGN_TYPED(offset, size, uint64_t);
-		error = dmu_buf_hold(os, object, offset, zgd, &db,
+		error = dmu_buf_hold_by_dnode(zv->zv_dn, offset, zgd, &db,
 		    DMU_READ_NO_PREFETCH);
 		if (error == 0) {
 			blkptr_t *obp = dmu_buf_get_blkptr(db);
@@ -1070,6 +1068,7 @@ zvol_setup_zv(zvol_state_t *zv)
 	int error;
 	uint64_t ro;
 	objset_t *os = zv->zv_objset;
+	dmu_buf_t *db;
 
 	error = dsl_prop_get_integer(zv->zv_name, "readonly", &ro, NULL);
 	if (error)
@@ -1079,7 +1078,7 @@ zvol_setup_zv(zvol_state_t *zv)
 	if (error)
 		return (SET_ERROR(error));
 
-	error = dmu_bonus_hold(os, ZVOL_OBJ, zv, &zv->zv_dbuf);
+	error = dnode_hold(os, ZVOL_OBJ, FTAG, &zv->zv_dn);
 	if (error)
 		return (SET_ERROR(error));
 
@@ -1108,8 +1107,8 @@ zvol_shutdown_zv(zvol_state_t *zv)
 	zil_close(zv->zv_zilog);
 	zv->zv_zilog = NULL;
 
-	dmu_buf_rele(zv->zv_dbuf, zv);
-	zv->zv_dbuf = NULL;
+	dnode_rele(zv->zv_dn, FTAG);
+	zv->zv_dn = NULL;
 
 	/*
 	 * Evict cached data