Index: share/man/man9/VOP_DEALLOCATE.9
===================================================================
--- share/man/man9/VOP_DEALLOCATE.9
+++ share/man/man9/VOP_DEALLOCATE.9
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 11, 2021
+.Dd August 11, 2021
 .Dt VOP_DEALLOCATE 9
 .Os
 .Sh NAME
@@ -42,6 +42,7 @@
 .Fa "off_t *offset"
 .Fa "off_t *len"
 .Fa "int flags"
+.Fa "int ioflag"
 .Fa "struct ucred *cred"
 .Fc
 .Sh DESCRIPTION
@@ -61,6 +62,8 @@
 .It Fa flags
 The flags of this call.
 This should be set to 0 for now.
+.It Fa ioflag
+Directives and hints to be given to the file system.
 .It Fa cred
 The credentials of the caller.
 .El
Index: share/man/man9/vn_deallocate.9
===================================================================
--- share/man/man9/vn_deallocate.9
+++ share/man/man9/vn_deallocate.9
@@ -67,7 +67,7 @@
 The control flags of the operation.
 This should be set to 0 for now.
 .It Fa ioflag
-The control flags of vnode locking.
+Various flag.
 .It Fa active_cred
 The user credentials of the calling thread.
 .It Fa file_cred
@@ -76,7 +76,8 @@
 .Pp
 The
 .Fn ioflag
-argument may be one or more of the following flags:
+argument give directives and hints to the file system.
+It may include one or more of the following flags:
 .Bl -tag -width IO_RANGELOCKED
 .It Dv IO_NODELOCKED
 The vnode was locked before the call.
@@ -84,6 +85,10 @@
 Rangelock was owned around the call.
 .It Dv IO_NOMACCHECK
 Skip MAC checking in the call.
+.It Dv IO_SYNC
+Do I/O synchronously.
+.It Dv IO_DIRECT
+Attempt to bypass buffer cache.
 .El
 .Pp
 .Fa *offset
Index: sys/kern/vfs_default.c
===================================================================
--- sys/kern/vfs_default.c
+++ sys/kern/vfs_default.c
@@ -1074,7 +1074,7 @@
 
 static int
 vp_zerofill(struct vnode *vp, struct vattr *vap, off_t *offsetp, off_t *lenp,
-    struct ucred *cred)
+    int ioflag, struct ucred *cred)
 {
 	int iosize;
 	int error = 0;
@@ -1110,7 +1110,7 @@
 		auio.uio_rw = UIO_WRITE;
 		auio.uio_td = td;
 
-		error = VOP_WRITE(vp, &auio, 0, cred);
+		error = VOP_WRITE(vp, &auio, ioflag, cred);
 		if (error != 0) {
 			len -= xfersize - auio.uio_resid;
 			offset += xfersize - auio.uio_resid;
@@ -1175,7 +1175,7 @@
 
 		/* Fill zeroes */
 		xfersize = rem = omin(noff - offset, len);
-		error = vp_zerofill(vp, &va, &offset, &rem, cred);
+		error = vp_zerofill(vp, &va, &offset, &rem, ap->a_ioflag, cred);
 		if (error) {
 			len -= xfersize - rem;
 			goto out;
Index: sys/kern/vfs_vnops.c
===================================================================
--- sys/kern/vfs_vnops.c
+++ sys/kern/vfs_vnops.c
@@ -916,6 +916,35 @@
 	return (ret);
 }
 
+static int
+get_write_ioflag(struct file *fp)
+{
+	int ioflag;
+	struct mount *mp;
+	struct vnode *vp;
+
+	ioflag = 0;
+	vp = fp->f_vnode;
+	mp = atomic_load_ptr(&vp->v_mount);
+
+	if (fp->f_flag & O_DIRECT)
+		ioflag |= IO_DIRECT;
+
+	if ((fp->f_flag & O_FSYNC) ||
+	    (mp != NULL && (mp->mnt_flag & MNT_SYNCHRONOUS)))
+		ioflag |= IO_SYNC;
+
+	/*
+	 * For O_DSYNC we set both IO_SYNC and IO_DATASYNC, so that VOP_WRITE()
+	 * or VOP_DEALLOCATE() implementations that don't understand IO_DATASYNC
+	 * fall back to full O_SYNC behavior.
+	 */
+	if (fp->f_flag & O_DSYNC)
+		ioflag |= IO_SYNC | IO_DATASYNC;
+
+	return (ioflag);
+}
+
 int
 vn_read_from_obj(struct vnode *vp, struct uio *uio)
 {
@@ -1119,21 +1148,8 @@
 		ioflag |= IO_APPEND;
 	if (fp->f_flag & FNONBLOCK)
 		ioflag |= IO_NDELAY;
-	if (fp->f_flag & O_DIRECT)
-		ioflag |= IO_DIRECT;
-
-	mp = atomic_load_ptr(&vp->v_mount);
-	if ((fp->f_flag & O_FSYNC) ||
-	    (mp != NULL && (mp->mnt_flag & MNT_SYNCHRONOUS)))
-		ioflag |= IO_SYNC;
+	ioflag |= get_write_ioflag(fp);
 
-	/*
-	 * For O_DSYNC we set both IO_SYNC and IO_DATASYNC, so that VOP_WRITE()
-	 * implementations that don't understand IO_DATASYNC fall back to full
-	 * O_SYNC behavior.
-	 */
-	if (fp->f_flag & O_DSYNC)
-		ioflag |= IO_SYNC | IO_DATASYNC;
 	mp = NULL;
 	need_finished_write = false;
 	if (vp->v_type != VCHR) {
@@ -3489,7 +3505,7 @@
 			    vp);
 #endif
 		if (error == 0)
-			error = VOP_DEALLOCATE(vp, &off, &len, flags,
+			error = VOP_DEALLOCATE(vp, &off, &len, flags, ioflag,
 			    active_cred);
 
 		if ((ioflag & IO_NODELOCKED) == 0) {
@@ -3528,6 +3544,7 @@
 {
 	int error;
 	struct vnode *vp;
+	int ioflag;
 
 	vp = fp->f_vnode;
 
@@ -3537,9 +3554,11 @@
 	if (vp->v_type != VREG)
 		return (ENODEV);
 
+	ioflag = get_write_ioflag(fp);
+
 	switch (cmd) {
 	case SPACECTL_DEALLOC:
-		error = vn_deallocate_impl(vp, offset, length, flags, 0,
+		error = vn_deallocate_impl(vp, offset, length, flags, ioflag,
 		    active_cred, fp->f_cred);
 		break;
 	default:
Index: sys/kern/vnode_if.src
===================================================================
--- sys/kern/vnode_if.src
+++ sys/kern/vnode_if.src
@@ -808,6 +808,7 @@
 	INOUT off_t *offset;
 	INOUT off_t *len;
 	IN int flags;
+	IN int ioflag;
 	IN struct ucred *cred;
 };