[RFC][PATCH 7/15] Union-mount mounting

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

From: Jan Blunck <j.blunck@tu-harburg.de>
Subject: Union-mount mounting

Adds union mount support to mount() and umount() system calls.
Sets up the union stack during mount and destroys it during unmount.

TODO: bind and move mounts aren't yet supported with union mounts.

Signed-off-by: Jan Blunck <j.blunck@tu-harburg.de>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 fs/namespace.c        |   79 +++++++++++++++++++++++++++++++++++++++++++++-----
 fs/union.c            |   65 +++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h    |    3 +
 include/linux/union.h |   33 ++++++++++++++++++++
 4 files changed, 173 insertions(+), 7 deletions(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -166,7 +166,7 @@ void mnt_set_mountpoint(struct vfsmount 
 			struct vfsmount *child_mnt)
 {
 	child_mnt->mnt_parent = mntget(mnt);
-	child_mnt->mnt_mountpoint = dget(dentry);
+	child_mnt->mnt_mountpoint = __dget(dentry);
 	dentry->d_mounted++;
 }
 
@@ -234,6 +234,10 @@ static struct vfsmount *clone_mnt(struct
 	struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);
 
 	if (mnt) {
+		/*
+		 * As of now, cloning of union mounted mnt isn't permitted.
+		 */
+		BUG_ON(mnt->mnt_flags & MNT_UNION);
 		mnt->mnt_flags = old->mnt_flags;
 		atomic_inc(&sb->s_active);
 		mnt->mnt_sb = sb;
@@ -522,16 +526,20 @@ void release_mounts(struct list_head *he
 		mnt = list_entry(head->next, struct vfsmount, mnt_hash);
 		list_del_init(&mnt->mnt_hash);
 		if (mnt->mnt_parent != mnt) {
-			struct dentry *dentry;
-			struct vfsmount *m;
+			struct path old_nd;
 			spin_lock(&vfsmount_lock);
-			dentry = mnt->mnt_mountpoint;
-			m = mnt->mnt_parent;
+			old_nd.dentry = mnt->mnt_mountpoint;
+			old_nd.mnt = mnt->mnt_parent;
 			mnt->mnt_mountpoint = mnt->mnt_root;
 			mnt->mnt_parent = mnt;
+			detach_mnt_union(mnt, &old_nd);
 			spin_unlock(&vfsmount_lock);
-			dput(dentry);
-			mntput(m);
+			if (mnt->mnt_flags & MNT_UNION) {
+				UM_DEBUG("shrink the mountpoint's dcache\n");
+				shrink_dcache_sb(old_nd.dentry->d_sb);
+			}
+			__dput(old_nd.dentry);
+			mntput(old_nd.mnt);
 		}
 		mntput(mnt);
 	}
@@ -564,6 +572,7 @@ static int do_umount(struct vfsmount *mn
 	struct super_block *sb = mnt->mnt_sb;
 	int retval;
 	LIST_HEAD(umount_list);
+	struct union_info *uinfo = NULL;
 
 	retval = security_sb_umount(mnt, flags);
 	if (retval)
@@ -628,6 +637,9 @@ static int do_umount(struct vfsmount *mn
 	}
 
 	down_write(&namespace_sem);
+	/* get the union lock which is released after release_mounts() */
+	if (mnt->mnt_flags & MNT_UNION)
+		uinfo = union_get2(mnt->mnt_root);
 	spin_lock(&vfsmount_lock);
 	event++;
 
@@ -642,6 +654,8 @@ static int do_umount(struct vfsmount *mn
 		security_sb_umount_busy(mnt);
 	up_write(&namespace_sem);
 	release_mounts(&umount_list);
+	if (uinfo)
+		union_release(uinfo);
 	return retval;
 }
 
@@ -857,6 +871,7 @@ static int attach_recursive_mnt(struct v
 		touch_mnt_namespace(current->nsproxy->mnt_ns);
 	} else {
 		mnt_set_mountpoint(dest_mnt, dest_dentry, source_mnt);
+		attach_mnt_union(source_mnt, nd);
 		commit_tree(source_mnt);
 	}
 
@@ -871,6 +886,8 @@ static int attach_recursive_mnt(struct v
 static int graft_tree(struct vfsmount *mnt, struct nameidata *nd)
 {
 	int err;
+	struct union_info *uinfo = NULL;
+
 	if (mnt->mnt_sb->s_flags & MS_NOUSER)
 		return -EINVAL;
 
@@ -878,6 +895,9 @@ static int graft_tree(struct vfsmount *m
 	      S_ISDIR(mnt->mnt_root->d_inode->i_mode))
 		return -ENOTDIR;
 
+	if (mnt->mnt_flags & MNT_UNION)
+		uinfo = union_alloc2(nd->dentry);
+
 	err = -ENOENT;
 	mutex_lock(&nd->dentry->d_inode->i_mutex);
 	if (IS_DEADDIR(nd->dentry->d_inode))
@@ -894,6 +914,8 @@ out_unlock:
 	mutex_unlock(&nd->dentry->d_inode->i_mutex);
 	if (!err)
 		security_sb_post_addmount(mnt, nd);
+	if (uinfo)
+		union_release(uinfo);
 	return err;
 }
 
@@ -909,6 +931,12 @@ static int do_change_type(struct nameida
 	if (nd->dentry != nd->mnt->mnt_root)
 		return -EINVAL;
 
+	/*
+	 * Don't change the type of union mounts
+	 */
+	if (nd->mnt->mnt_flags & MNT_UNION)
+		return -EINVAL;
+
 	down_write(&namespace_sem);
 	spin_lock(&vfsmount_lock);
 	for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
@@ -934,6 +962,15 @@ static int do_loopback(struct nameidata 
 	if (err)
 		return err;
 
+	/*
+	 * bind mounting to or from union mounts is not supported
+	 */
+	err = -EINVAL;
+	if (nd->mnt->mnt_flags & MNT_UNION)
+		goto out_unlocked;
+	if (old_nd.mnt->mnt_flags & MNT_UNION)
+		goto out_unlocked;
+
 	down_write(&namespace_sem);
 	err = -EINVAL;
 	if (IS_MNT_UNBINDABLE(old_nd.mnt))
@@ -962,6 +999,7 @@ static int do_loopback(struct nameidata 
 
 out:
 	up_write(&namespace_sem);
+out_unlocked:
 	path_release(&old_nd);
 	return err;
 }
@@ -1019,6 +1057,15 @@ static int do_move_mount(struct nameidat
 	if (err)
 		return err;
 
+	/*
+	 * moving to or from a union mount is not supported
+	 */
+	err = -EINVAL;
+	if (nd->mnt->mnt_flags & MNT_UNION)
+		goto exit;
+	if (old_nd.mnt->mnt_flags & MNT_UNION)
+		goto exit;
+
 	down_write(&namespace_sem);
 	while (d_mountpoint(nd->dentry) && follow_down(&nd->mnt, &nd->dentry))
 		;
@@ -1074,6 +1121,7 @@ out:
 	up_write(&namespace_sem);
 	if (!err)
 		path_release(&parent_nd);
+exit:
 	path_release(&old_nd);
 	return err;
 }
@@ -1098,6 +1146,9 @@ static int do_new_mount(struct nameidata
 	if (IS_ERR(mnt))
 		return PTR_ERR(mnt);
 
+	UM_DEBUG("dentry=%s, device=%s\n", nd->dentry->d_name.name,
+	       mnt->mnt_devname);
+
 	return do_add_mount(mnt, nd, mnt_flags, NULL);
 }
 
@@ -1128,6 +1179,12 @@ int do_add_mount(struct vfsmount *newmnt
 	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
 		goto unlock;
 
+	/* Unions couldn't include shared mounts */
+	err = -EINVAL;
+	if ((mnt_flags & MNT_UNION) &&
+	    IS_MNT_SHARED(nd->mnt))
+		goto unlock;
+
 	/* Unions couldn't be writable if the filesystem
 	 * doesn't know about whiteouts */
 	err = -ENOTSUPP;
@@ -1146,6 +1203,14 @@ int do_add_mount(struct vfsmount *newmnt
 		list_add_tail(&newmnt->mnt_expire, fslist);
 		spin_unlock(&vfsmount_lock);
 	}
+
+	UM_DEBUG("mntpoint->d_count=%d/%p\n",
+		 atomic_read(&nd->dentry->d_count),
+		 &nd->dentry->d_count);
+	UM_DEBUG("mntroot->d_count=%d/%p\n",
+		 atomic_read(&newmnt->mnt_root->d_count),
+		 &newmnt->mnt_root->d_count);
+
 	up_write(&namespace_sem);
 	return 0;
 
--- a/fs/union.c
+++ b/fs/union.c
@@ -300,3 +300,68 @@ void __dput_union(struct dentry *dentry)
 
 	return;
 }
+
+void attach_mnt_union(struct vfsmount *mnt, struct nameidata *nd)
+{
+	struct dentry *tmp;
+
+	if (!(mnt->mnt_flags & MNT_UNION))
+		return;
+
+	UM_DEBUG("MNT_UNION set for dentry \"%s\", devname=%s\n",
+		 mnt->mnt_root->d_name.name, mnt->mnt_devname);
+	UM_DEBUG("mountpoint \"%s\", inode=%p\n",
+		 nd->dentry->d_name.name, nd->dentry->d_inode);
+
+	mnt->mnt_root->d_overlaid = __dget(nd->dentry);
+	mnt->mnt_root->d_topmost = NULL;
+	mnt->mnt_root->d_union = union_get(nd->dentry->d_union);
+
+	tmp = nd->dentry;
+	while (tmp) {
+		tmp->d_topmost = mnt->mnt_root;
+		tmp = tmp->d_overlaid;
+	}
+}
+
+void detach_mnt_union(struct vfsmount *mnt, struct path *path)
+{
+	struct dentry *tmp;
+
+	if (!(mnt->mnt_flags & MNT_UNION))
+		return;
+
+	UM_DEBUG("MNT_UNION set for dentry \"%s\", devname=%s\n",
+		 mnt->mnt_root->d_name.name, mnt->mnt_devname);
+	UM_DEBUG("mountpoint \"%s\", inode=%p\n",
+		 path->dentry->d_name.name, path->dentry->d_inode);
+	BUG_ON(mnt->mnt_root->d_topmost);
+
+	/* put reference to the underlying union stack */
+	__dput(mnt->mnt_root->d_overlaid);
+	mnt->mnt_root->d_overlaid = NULL;
+	union_put(mnt->mnt_root->d_union);
+	mnt->mnt_root->d_union = NULL;
+
+	/* rearrange the union stack */
+	path->dentry->d_topmost = NULL;
+	tmp = path->dentry->d_overlaid;
+	while (tmp) {
+		tmp->d_topmost = path->dentry;
+		tmp = tmp->d_overlaid;
+	}
+
+	/* If the mount point is the last component in the union,
+	 * put the reference to the union struct */
+	if (!path->dentry->d_overlaid) {
+		union_put(path->dentry->d_union);
+		path->dentry->d_union = NULL;
+	}
+
+	/* when we looked up the mountpoint to be unmounted
+	 * we dget() a union-mount dentry struct so we have
+	 * to dput() parts of it by hand before we remove the
+	 * topmost dentry (which is mnt->mnt_root) from the
+	 * union stack */
+	__dput(path->dentry);
+}
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1976,6 +1976,9 @@ static inline ino_t parent_ino(struct de
 /* kernel/fork.c */
 extern int unshare_files(void);
 
+/* fs/union.c */
+#include <linux/union.h>
+
 /* Transaction based IO helpers */
 
 /*
--- /dev/null
+++ b/include/linux/union.h
@@ -0,0 +1,33 @@
+/*
+ * VFS based union mount for Linux
+ *
+ * Copyright © 2004-2007 IBM Corporation
+ *   Author(s): Jan Blunck (j.blunck@tu-harburg.de)
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ */
+#ifndef __LINUX_UNION_H
+#define __LINUX_UNION_H
+#ifdef __KERNEL__
+
+#ifdef CONFIG_UNION_MOUNT
+
+#include <linux/fs_struct.h>
+
+/* namespace stuff used at mount time */
+extern void attach_mnt_union(struct vfsmount *, struct nameidata *);
+extern void detach_mnt_union(struct vfsmount *, struct path *);
+
+#else	/* CONFIG_UNION_MOUNT */
+
+#define attach_mnt_union(mnt,nd) do { /* empty */ } while (0)
+#define detach_mnt_union(mnt,nd) do { /* empty */ } while (0)
+
+#endif	/* CONFIG_UNION_MOUNT */
+
+#endif	/* __KERNEL __ */
+#endif	/* __LINUX_UNION_H */
-
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