Create an 'overlay' property

Add a new 'overlay' property (default 'off') that controls whether the
filesystem should be mounted even if the mountpoint is busy or if it
should fail with a 'mountpoint not empty'.

Doing overlay mounts is the default mount behavior on Linux, but not
in ZFS. It have been decided that following the ZFS behavior should
be the default, but this overlay allows for site administrator to
override this decision on a per-dataset basis.

Signed-off-by: Turbo Fredriksson <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes: #2503

4 files changed, 28 insertions, 0 deletions

diff --git a/cmd/zfs/zfs_main.c b/cmd/zfs/zfs_main.c
index 84073435e..83f02666d 100644
--- a/cmd/zfs/zfs_main.c
+++ b/cmd/zfs/zfs_main.c
@@ -5641,6 +5641,7 @@ share_mount_one(zfs_handle_t *zhp, int op, int flags, char *protocol,
 	char mountpoint[ZFS_MAXPROPLEN];
 	char shareopts[ZFS_MAXPROPLEN];
 	char smbshareopts[ZFS_MAXPROPLEN];
+	char overlay[ZFS_MAXPROPLEN];
 	const char *cmdname = op == OP_SHARE ? "share" : "mount";
 	struct mnttab mnt;
 	uint64_t zoned, canmount;
@@ -5748,6 +5749,19 @@ share_mount_one(zfs_handle_t *zhp, int op, int flags, char *protocol,
 	}
 
 	/*
+	 * Overlay mounts are disabled by default but may be enabled
+	 * via the 'overlay' property or the 'zfs mount -O' option.
+	 */
+	if (!(flags & MS_OVERLAY)) {
+		if (zfs_prop_get(zhp, ZFS_PROP_OVERLAY, overlay,
+			    sizeof (overlay), NULL, NULL, 0, B_FALSE) == 0) {
+			if (strcmp(overlay, "on") == 0) {
+				flags |= MS_OVERLAY;
+			}
+		}
+	}
+
+	/*
 	 * At this point, we have verified that the mountpoint and/or
 	 * shareopts are appropriate for auto management. If the
 	 * filesystem is already mounted or shared, return (failing
diff --git a/include/sys/fs/zfs.h b/include/sys/fs/zfs.h
index 227d8b2fb..d54cd5679 100644
--- a/include/sys/fs/zfs.h
+++ b/include/sys/fs/zfs.h
@@ -150,6 +150,7 @@ typedef enum {
 	ZFS_PROP_SELINUX_ROOTCONTEXT,
 	ZFS_PROP_RELATIME,
 	ZFS_PROP_REDUNDANT_METADATA,
+	ZFS_PROP_OVERLAY,
 	ZFS_NUM_PROPS
 } zfs_prop_t;
 
diff --git a/man/man8/zfs.8 b/man/man8/zfs.8
index 96cac6f5a..8b22c5af4 100644
--- a/man/man8/zfs.8
+++ b/man/man8/zfs.8
@@ -1409,6 +1409,17 @@ This flag sets the SELinux context for unlabeled files.  See \fBselinux\fR(8) fo
 This flag sets the SELinux context for the root inode of the filesystem.  See \fBselinux\fR(8) for more information.
 .RE
 
+.sp
+.ne 2
+.mk
+.na
+\fB\fBoverlay\fR=\fBon\fR | \fBoff\fR\fR
+.ad
+.sp .6
+.RS 4n
+Allow mounting on a busy directory or a directory which already contains files/directories. This is the default mount behavior for Linux filesystems.  However, for consistency with ZFS on other platforms overlay mounts are disabled by default.  Set \fBoverlay=on\fR to enable overlay mounts.
+.RE
+
 .SS "Temporary Mount Point Properties"
 .LP
 When a file system is mounted, either through \fBmount\fR(8) for legacy mounts or the \fBzfs mount\fR command for normal file systems, its mount options are set according to its properties. The correlation between properties and mount options is as follows:
diff --git a/module/zcommon/zfs_prop.c b/module/zcommon/zfs_prop.c
index 77491cdb7..e60c4f7d4 100644
--- a/module/zcommon/zfs_prop.c
+++ b/module/zcommon/zfs_prop.c
@@ -295,6 +295,8 @@ zfs_prop_init(void)
 	zprop_register_index(ZFS_PROP_NBMAND, "nbmand", 0, PROP_INHERIT,
 	    ZFS_TYPE_FILESYSTEM | ZFS_TYPE_SNAPSHOT, "on | off", "NBMAND",
 	    boolean_table);
+	zprop_register_index(ZFS_PROP_OVERLAY, "overlay", 0, PROP_INHERIT,
+	    ZFS_TYPE_FILESYSTEM, "on | off", "OVERLAY", boolean_table);
 
 	/* default index properties */
 	zprop_register_index(ZFS_PROP_VERSION, "version", 0, PROP_DEFAULT,