Allow mounting datasets more than once

config/kernel-fst-mount.m4

@@ -0,0 +1,28 @@
+dnl #
+dnl # 2.6.38 API change
+dnl # The .get_sb callback has been replaced by a .mount callback
+dnl # in the file_system_type structure.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_FST_MOUNT], [
+        AC_MSG_CHECKING([whether fst->mount() exists])
+        ZFS_LINUX_TRY_COMPILE([
+                #include <linux/fs.h>
+
+                static struct dentry *
+                mount(struct file_system_type *fs_type, int flags,
+                    const char *osname, void *data) {
+                        struct dentry *d = NULL;
+                        return (d);
+                }
+
+                static struct file_system_type fst __attribute__ ((unused)) = {
+                        .mount = mount,
+                };
+        ],[
+        ],[
+                AC_MSG_RESULT(yes)
+                AC_DEFINE(HAVE_FST_MOUNT, 1, [fst->mount() exists])
+        ],[
+                AC_MSG_RESULT(no)
+        ])
+])

config/kernel.m4

@@ -101,7 +101,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_6ARGS_SECURITY_INODE_INIT_SECURITY
 	ZFS_AC_KERNEL_CALLBACK_SECURITY_INODE_INIT_SECURITY
-	ZFS_AC_KERNEL_MOUNT_NODEV
+	ZFS_AC_KERNEL_FST_MOUNT
 	ZFS_AC_KERNEL_SHRINK
 	ZFS_AC_KERNEL_SHRINK_CONTROL_HAS_NID
 	ZFS_AC_KERNEL_S_INSTANCES_LIST_HEAD

include/linux/vfs_compat.h

@@ -182,6 +182,30 @@ zpl_bdi_destroy(struct super_block *sb)
 }
 #endif
 
+/*
+ * 4.14 adds SB_* flag definitions, define them to MS_* equivalents
+ * if not set.
+ */
+#ifndef	SB_RDONLY
+#define	SB_RDONLY	MS_RDONLY
+#endif
+
+#ifndef	SB_SILENT
+#define	SB_SILENT	MS_SILENT
+#endif
+
+#ifndef	SB_ACTIVE
+#define	SB_ACTIVE	MS_ACTIVE
+#endif
+
+#ifndef	SB_POSIXACL
+#define	SB_POSIXACL	MS_POSIXACL
+#endif
+
+#ifndef	SB_MANDLOCK
+#define	SB_MANDLOCK	MS_MANDLOCK
+#endif
+
 /*
  * 2.6.38 API change,
  * LOOKUP_RCU flag introduced to distinguish rcu-walk from ref-walk cases.

module/zfs/zpl_super.c

@@ -248,14 +248,61 @@ zpl_fill_super(struct super_block *sb, void *data, int silent)
 	return (error);
 }
 
-#ifdef HAVE_MOUNT_NODEV
+static int
+zpl_test_super(struct super_block *s, void *data)
+{
+	zfsvfs_t *zfsvfs = s->s_fs_info;
+	objset_t *os = data;
+
+	if (zfsvfs == NULL)
+		return (0);
+
+	return (os == zfsvfs->z_os);
+}
+
+static struct super_block *
+zpl_mount_impl(struct file_system_type *fs_type, int flags, zfs_mnt_t *zm)
+{
+	struct super_block *s;
+	objset_t *os;
+	int err;
+
+	err = dmu_objset_hold(zm->mnt_osname, FTAG, &os);
+	if (err)
+		return (ERR_PTR(-err));
+
+	s = zpl_sget(fs_type, zpl_test_super, set_anon_super, flags, os);
+	dmu_objset_rele(os, FTAG);
+	if (IS_ERR(s))
+		return (ERR_CAST(s));
+
+	if (s->s_root == NULL) {
+		err = zpl_fill_super(s, zm, flags & SB_SILENT ? 1 : 0);
+		if (err) {
+			deactivate_locked_super(s);
+			return (ERR_PTR(err));
+		}
+		s->s_flags |= SB_ACTIVE;
+	} else if ((flags ^ s->s_flags) & SB_RDONLY) {
+		deactivate_locked_super(s);
+		return (ERR_PTR(-EBUSY));
+	}
+
+	return (s);
+}
+
+#ifdef HAVE_FST_MOUNT
 static struct dentry *
 zpl_mount(struct file_system_type *fs_type, int flags,
     const char *osname, void *data)
 {
 	zfs_mnt_t zm = { .mnt_osname = osname, .mnt_data = data };
 
-	return (mount_nodev(fs_type, flags, &zm, zpl_fill_super));
+	struct super_block *sb = zpl_mount_impl(fs_type, flags, &zm);
+	if (IS_ERR(sb))
+		return (ERR_CAST(sb));
+
+	return (dget(sb->s_root));
 }
 #else
 static int
@@ -264,9 +311,15 @@ zpl_get_sb(struct file_system_type *fs_type, int flags,
 {
 	zfs_mnt_t zm = { .mnt_osname = osname, .mnt_data = data };
 
-	return (get_sb_nodev(fs_type, flags, &zm, zpl_fill_super, mnt));
+	struct super_block *sb = zpl_mount_impl(fs_type, flags, &zm);
+	if (IS_ERR(sb))
+		return (PTR_ERR(sb));
+
+	(void) simple_set_mnt(mnt, sb);
+
+	return (0);
 }
-#endif /* HAVE_MOUNT_NODEV */
+#endif /* HAVE_FST_MOUNT */
 
 static void
 zpl_kill_sb(struct super_block *sb)
@@ -333,10 +386,10 @@ const struct super_operations zpl_super_operations = {
 struct file_system_type zpl_fs_type = {
 	.owner			= THIS_MODULE,
 	.name			= ZFS_DRIVER,
-#ifdef HAVE_MOUNT_NODEV
+#ifdef HAVE_FST_MOUNT
 	.mount			= zpl_mount,
 #else
 	.get_sb			= zpl_get_sb,
-#endif /* HAVE_MOUNT_NODEV */
+#endif /* HAVE_FST_MOUNT */
 	.kill_sb		= zpl_kill_sb,
 };

tests/runfiles/linux.run

@@ -174,7 +174,7 @@ tests = ['zfs_mount_001_pos', 'zfs_mount_002_pos', 'zfs_mount_003_pos',
     'zfs_mount_004_pos', 'zfs_mount_005_pos', 'zfs_mount_007_pos',
     'zfs_mount_008_pos', 'zfs_mount_009_neg', 'zfs_mount_010_neg',
     'zfs_mount_011_neg', 'zfs_mount_012_neg', 'zfs_mount_all_001_pos',
-    'zfs_mount_encrypted', 'zfs_mount_remount']
+    'zfs_mount_encrypted', 'zfs_mount_remount', 'zfs_multi_mount']
 tags = ['functional', 'cli_root', 'zfs_mount']
 
 [tests/functional/cli_root/zfs_program]

tests/zfs-tests/tests/functional/cli_root/zfs_mount/Makefile.am

@@ -16,7 +16,8 @@ dist_pkgdata_SCRIPTS = \
 	zfs_mount_012_neg.ksh \
 	zfs_mount_encrypted.ksh \
 	zfs_mount_all_001_pos.ksh \
-	zfs_mount_remount.ksh
+	zfs_mount_remount.ksh \
+	zfs_multi_mount.ksh
 
 dist_pkgdata_DATA = \
 	zfs_mount.cfg \

tests/zfs-tests/tests/functional/cli_root/zfs_mount/zfs_multi_mount.ksh

@@ -0,0 +1,104 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy is of the CDDL is also available via the Internet
+# at http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright(c) 2018 Datto Inc.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# Verify multi mount functionality
+#
+# STRATEGY:
+# 1. Create fs
+# 2. Create and hold open file in filesystem
+# 3. Lazy unmount
+# 4. Verify remounting fs that was lazily unmounted is possible
+# 5. Verify multiple mounts of the same dataset are possible
+# 6. Verify bind mount doesn't prevent rename
+#
+
+verify_runnable "both"
+
+function cleanup
+{
+	ismounted $MNTPFS && log_must umount $MNTPFS
+	ismounted $MNTPFS2 && log_must umount $MNTPFS2
+	ismounted $MNTPFS3 && log_must umount $MNTPFS3
+	ismounted $MNTPFS4 && log_must umount $MNTPFS4
+	ismounted $RENAMEMNT && log_must umount $RENAMEMNT
+	datasetexists $TESTDS && log_must destroy_dataset "$TESTDS" "-f"
+}
+log_onexit cleanup
+
+log_assert "Verify multiple mounts into one namespace are possible"
+
+# 1. Create fs
+TESTDS="$TESTPOOL/multi-mount-test"
+log_must zfs create $TESTDS
+
+# 2. Create and hold open file in filesystem
+MNTPFS="$(get_prop mountpoint $TESTDS)"
+FILENAME="$MNTPFS/file"
+log_must mkfile 128k $FILENAME
+log_must exec 9<> $FILENAME # open file
+
+# 3. Lazy umount
+log_must umount -l $MNTPFS
+if [ -f $FILENAME ]; then
+	log_fail "Lazy unmount failed"
+fi
+
+# 4. Verify remounting fs that was lazily unmounted is possible
+log_must zfs mount $TESTDS
+if [ ! -f $FILENAME ]; then
+	log_fail "Lazy remount failed"
+fi
+log_must exec 9>&- # close fd
+
+# 5. Verify multiple mounts of the same dataset are possible
+MNTPFS2="$MNTPFS-second"
+FILENAME="$MNTPFS2/file"
+log_must mkdir $MNTPFS2
+log_must mount -t zfs -o zfsutil $TESTDS $MNTPFS2
+if [ ! -f $FILENAME ]; then
+	log_fail "First multi mount failed"
+fi
+
+MNTPFS3="$MNTPFS-third"
+FILENAME="$MNTPFS3/file"
+log_must mkdir $MNTPFS3
+log_must mount -t zfs -o zfsutil $TESTDS $MNTPFS3
+if [ ! -f $FILENAME ]; then
+	log_fail "Second multi mount failed"
+fi
+
+# 6. Verify bind mount doesn't prevent rename
+RENAMEFS="$TESTDS-newname"
+MNTPFS4="$MNTPFS-fourth"
+log_must mkdir $MNTPFS4
+log_must mount --bind $MNTPFS $MNTPFS4
+log_must zfs rename $TESTDS $RENAMEFS
+RENAMEMNT="$(get_prop mountpoint $RENAMEFS)"
+FILENAME="$RENAMEMNT/file"
+if [ ! -f $FILENAME ]; then
+	log_fail "Rename failed"
+fi
+log_must zfs rename $RENAMEFS $TESTDS
+
+log_pass "Multiple mounts are possible"