Index: sbin/mdconfig/mdconfig.8
===================================================================
--- sbin/mdconfig/mdconfig.8
+++ sbin/mdconfig/mdconfig.8
@@ -37,7 +37,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 6, 2020
+.Dd August 27, 2021
 .Dt MDCONFIG 8
 .Os
 .Sh NAME
@@ -256,6 +256,12 @@
 The type of verification depends on which security features are available.
 One example of verification is testing file integrity with
 checksums or cryptographic signatures.
+.It Oo Cm no Oc Ns Cm autounmap
+Detect if hole-punching is supported by the underlying file system.
+If the underlying file system of the vnode type memory disk does not support
+hole-punching,
+.Dv BIO_DELETE
+requests are returned unsupported.
 .El
 .It Fl u Ar unit
 Request a specific unit number or device name for the
Index: sbin/mdconfig/mdconfig.c
===================================================================
--- sbin/mdconfig/mdconfig.c
+++ sbin/mdconfig/mdconfig.c
@@ -88,8 +88,8 @@
 "       mdconfig -l [-v] [-n] [-f file] [-u unit]\n"
 "       mdconfig file\n");
 	fprintf(stderr, "\t\ttype = {malloc, vnode, swap}\n");
-	fprintf(stderr, "\t\toption = {cache, cluster, compress, force,\n");
-	fprintf(stderr, "\t\t          readonly, reserve, ro, verify}\n");
+	fprintf(stderr, "\t\toption = {autounmap, cache, cluster, compress,\n");
+	fprintf(stderr, "\t\t          force, readonly, reserve, ro, verify}\n");
 	fprintf(stderr, "\t\tsize = %%d (512 byte blocks), %%db (B),\n");
 	fprintf(stderr, "\t\t       %%dk (kB), %%dm (MB), %%dg (GB), \n");
 	fprintf(stderr, "\t\t       %%dt (TB), or %%dp (PB)\n");
@@ -210,6 +210,10 @@
 				mdio.md_options |= MD_VERIFY;
 			else if (!strcmp(optarg, "noverify"))
 				mdio.md_options &= ~MD_VERIFY;
+			else if (!strcmp(optarg, "autounmap"))
+				mdio.md_options |= MD_AUTOUNMAP;
+			else if (!strcmp(optarg, "noautounmap"))
+				mdio.md_options &= ~MD_AUTOUNMAP;
 			else
 				errx(1, "unknown option: %s", optarg);
 			break;
Index: sys/dev/md/md.c
===================================================================
--- sys/dev/md/md.c
+++ sys/dev/md/md.c
@@ -90,6 +90,7 @@
 #include <sys/sf_buf.h>
 #include <sys/sysctl.h>
 #include <sys/uio.h>
+#include <sys/unistd.h>
 #include <sys/vnode.h>
 #include <sys/disk.h>
 
@@ -259,6 +260,7 @@
 	struct g_provider *pp;
 	int (*start)(struct md_s *sc, struct bio *bp);
 	struct devstat *devstat;
+	bool candelete;
 
 	/* MD_MALLOC related fields */
 	struct indir *indir;
@@ -885,9 +887,12 @@
 	case BIO_WRITE:
 		auio.uio_rw = UIO_WRITE;
 		break;
-	case BIO_DELETE:
 	case BIO_FLUSH:
 		break;
+	case BIO_DELETE:
+		if (sc->candelete)
+			break;
+		/* FALLTHROUGH */
 	default:
 		return (EOPNOTSUPP);
 	}
@@ -1176,7 +1181,7 @@
 	    (g_handleattr_int(bp, "GEOM::fwsectors", sc->fwsectors) != 0 ||
 	    g_handleattr_int(bp, "GEOM::fwheads", sc->fwheads) != 0))
 		return;
-	if (g_handleattr_int(bp, "GEOM::candelete", 1) != 0)
+	if (g_handleattr_int(bp, "GEOM::candelete", sc->candelete) != 0)
 		return;
 	if (sc->ident[0] != '\0' &&
 	    g_handleattr_str(bp, "GEOM::ident", sc->ident) != 0)
@@ -1405,6 +1410,7 @@
 	struct nameidata nd;
 	char *fname;
 	int error, flags;
+	long v;
 
 	fname = mdr->md_file;
 	if (mdr->md_file_seg == UIO_USERSPACE) {
@@ -1434,6 +1440,13 @@
 	error = VOP_GETATTR(nd.ni_vp, &vattr, td->td_ucred);
 	if (error != 0)
 		goto bad;
+	if ((mdr->md_options & MD_AUTOUNMAP) != 0) {
+		error = VOP_PATHCONF(nd.ni_vp, _PC_DEALLOC_PRESENT, &v);
+		if (error != 0)
+			goto bad;
+		if (v == 0)
+			sc->candelete = false;
+	}
 	if (VOP_ISLOCKED(nd.ni_vp) != LK_EXCLUSIVE) {
 		vn_lock(nd.ni_vp, LK_UPGRADE | LK_RETRY);
 		if (VN_IS_DOOMED(nd.ni_vp)) {
@@ -1690,6 +1703,7 @@
 		mdr->md_unit = sc->unit;
 	sc->mediasize = mdr->md_mediasize;
 	sc->sectorsize = sectsize;
+	sc->candelete = true;
 	error = EDOOFUS;
 	switch (sc->type) {
 	case MD_MALLOC:
Index: sys/sys/mdioctl.h
===================================================================
--- sys/sys/mdioctl.h
+++ sys/sys/mdioctl.h
@@ -92,5 +92,6 @@
 #define MD_ASYNC	0x40	/* Asynchronous mode */
 #define MD_VERIFY	0x80	/* Open file with O_VERIFY (vnode only) */
 #define	MD_CACHE	0x100	/* Cache vnode data */
+#define	MD_AUTOUNMAP	0x200	/* Detect if hole-punching is available */
 
 #endif	/* _SYS_MDIOCTL_H_*/