Index: head/sys/kern/vfs_subr.c
===================================================================
--- head/sys/kern/vfs_subr.c
+++ head/sys/kern/vfs_subr.c
@@ -1746,8 +1746,6 @@
 		    ((flags & V_ALT) && (bp->b_xflags & BX_ALTDATA) == 0)) {
 			continue;
 		}
-		lblkno = 0;
-		xflags = 0;
 		if (nbp != NULL) {
 			lblkno = nbp->b_lblkno;
 			xflags = nbp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN);
@@ -1782,6 +1780,8 @@
 		bp->b_flags &= ~B_ASYNC;
 		brelse(bp);
 		BO_LOCK(bo);
+		if (nbp == NULL)
+			break;
 		nbp = gbincore(bo, lblkno);
 		if (nbp == NULL || (nbp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN))
 		    != xflags)