[PATCH] introduce shared mounts

This creates shared mounts.  A shared mount when bind-mounted to some
mountpoint, propagates mount/umount events to each other.  All the
shared mounts that propagate events to each other belong to the same
peer-group.
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/namespace.c

@@ -68,6 +68,7 @@ struct vfsmount *alloc_vfsmnt(const char *name)
 		INIT_LIST_HEAD(&mnt->mnt_mounts);
 		INIT_LIST_HEAD(&mnt->mnt_list);
 		INIT_LIST_HEAD(&mnt->mnt_expire);
+		INIT_LIST_HEAD(&mnt->mnt_share);
 		if (name) {
 			int size = strlen(name) + 1;
 			char *newname = kmalloc(size, GFP_KERNEL);
@@ -1113,7 +1114,7 @@ long do_mount(char *dev_name, char *dir_name, char *type_page,
 				    data_page);
 	else if (flags & MS_BIND)
 		retval = do_loopback(&nd, dev_name, flags & MS_REC);
-	else if (flags & MS_PRIVATE)
+	else if (flags & (MS_SHARED | MS_PRIVATE))
 		retval = do_change_type(&nd, flags);
 	else if (flags & MS_MOVE)
 		retval = do_move_mount(&nd, dev_name);

fs/pnode.c

@@ -11,7 +11,18 @@
 #include <linux/fs.h>
 #include "pnode.h"
 
+/* return the next shared peer mount of @p */
+static inline struct vfsmount *next_peer(struct vfsmount *p)
+{
+	return list_entry(p->mnt_share.next, struct vfsmount, mnt_share);
+}
+
 void change_mnt_propagation(struct vfsmount *mnt, int type)
 {
-	mnt->mnt_flags &= ~MNT_PNODE_MASK;
+	if (type == MS_SHARED) {
+		mnt->mnt_flags |= MNT_SHARED;
+	} else {
+		list_del_init(&mnt->mnt_share);
+		mnt->mnt_flags &= ~MNT_PNODE_MASK;
+	}
 }

fs/pnode.h

@@ -10,5 +10,9 @@
 
 #include <linux/list.h>
 #include <linux/mount.h>
+
+#define IS_MNT_SHARED(mnt) (mnt->mnt_flags & MNT_SHARED)
+#define CLEAR_MNT_SHARED(mnt) (mnt->mnt_flags &= ~MNT_SHARED)
+
 void change_mnt_propagation(struct vfsmount *, int);
 #endif /* _LINUX_PNODE_H */

include/linux/fs.h

@@ -105,6 +105,7 @@ extern int dir_notify_enable;
 #define MS_REC		16384
 #define MS_VERBOSE	32768
 #define MS_PRIVATE	(1<<18)	/* change to private */
+#define MS_SHARED	(1<<20)	/* change to shared */
 #define MS_POSIXACL	(1<<16)	/* VFS does not apply the umask */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)

include/linux/mount.h

@@ -20,6 +20,7 @@
 #define MNT_NOSUID	0x01
 #define MNT_NODEV	0x02
 #define MNT_NOEXEC	0x04
+#define MNT_SHARED	0x10	/* if the vfsmount is a shared mount */
 #define MNT_PNODE_MASK	0x30	/* propogation flag mask */
 
 struct vfsmount {
@@ -36,6 +37,7 @@ struct vfsmount {
 	char *mnt_devname;		/* Name of device e.g. /dev/dsk/hda1 */
 	struct list_head mnt_list;
 	struct list_head mnt_expire;	/* link in fs-specific expiry list */
+	struct list_head mnt_share;	/* circular list of shared mounts */
 	struct namespace *mnt_namespace; /* containing namespace */
 	int mnt_pinned;
 };