=> openzfs#2388: ryao/recv - ZFS_IOC_RECV should only call zvol_creat…

…e_minors() on zvols.

ZFS_IOC_RECV should only call zvol_create_minors() on zvols

perf profiling revealed significant time spent in these functions on a
system without zvols during a recv. I know of one company that is
explicitly disabling this in their own builds because they found that it
wastes CPU time. This is the intermediate approach.

Signed-off-by: Richard Yao <ryao@gentoo.org>

module/zfs/zfs_ioctl.c

@@ -4171,8 +4171,17 @@ zfs_ioc_recv(zfs_cmd_t *zc)
 #endif
 
 #ifdef _KERNEL
-	if (error == 0)
-		zvol_create_minors(tofs);
+	if (error == 0) {
+		objset_t *os;
+		boolean_t iszvol;
+
+		dmu_objset_hold(tofs, FTAG, &os);
+		iszvol = (dmu_objset_type(os) == DMU_OST_ZVOL);
+		dmu_objset_rele(os, FTAG);
+
+		if (iszvol)
+			zvol_create_minors(tofs);
+	}
 #endif
 
 	/*