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Subject[PATCH 19/34] union-mount: Add clone_union_tree() and put_union_sb()
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A union mount clones the vfsmount tree of all of the read-only layers
of the union and keeps a reference to it in the vfsmount of the
topmost layer of the union.

clone_union_tree() takes the path of the proposed union mountpoint and
attempts to clones every vfsmount mounted at that same pathname, as
well as their submounts. All these mounts must be read-only, not
slave, and not shared.

put_union_sb() unwinds everything clone_union_tree() does. It is
called when the superblock is deactivated. Thus, you can lazy unmount
a union mount and when the last reference goes away, the union will be
torn down.

Signed-off-by: Valerie Aurora <vaurora@redhat.com>
---
fs/namespace.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/mount.h | 2 +
2 files changed, 73 insertions(+), 0 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 62c2f7a..e966890 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1393,6 +1393,77 @@ static int check_topmost_union_mnt(struct vfsmount *topmost_mnt, int mnt_flags)
return 0;
}

+void put_union_sb(struct super_block *sb)
+{
+ struct vfsmount *mnt = sb->s_union_lower_mnts;
+ LIST_HEAD(umount_list);
+
+ if (!mnt)
+ return;
+ spin_lock(&vfsmount_lock);
+ umount_tree(mnt, 0, &umount_list);
+ spin_unlock(&vfsmount_lock);
+ release_mounts(&umount_list);
+ sb->s_union_lower_mnts = 0;
+ sb->s_union_count = 0;
+}
+
+/**
+ * clone_union_tree - Clone all union-able mounts at this mountpoint
+ *
+ * @topmost - vfsmount of topmost layer
+ * @mntpnt - target of union mount
+ *
+ * Given the target mountpoint of a union mount, clone all the mounts
+ * at that mountpoint (well, pathname) that qualify as a union lower
+ * layer. Increment the hard readonly count of the lower layer
+ * superblocks.
+ *
+ * Returns error if any of the mounts or submounts mounted on or below
+ * this pathname are unsuitable for union mounting. This means you
+ * can't construct a union mount at the root of an existing mount
+ * without unioning it.
+ *
+ * XXX - Maybe should take # of layers to go down as an argument. But
+ * how to pass this in through mount options? All solutions look
+ * ugly. Currently you express your intention through mounting file
+ * systems on the same mountpoint, which is pretty elegant.
+ */
+
+static int clone_union_tree(struct vfsmount *topmost, struct path *mntpnt)
+{
+ struct vfsmount *mnt, *cloned_tree;
+
+ if (!IS_ROOT(mntpnt->dentry)) {
+ printk(KERN_INFO "union mount: mount point must be a root dir\n");
+ return -EINVAL;
+ }
+
+ /* Look for the "lowest" layer to union. */
+ mnt = mntpnt->mnt;
+ while (mnt->mnt_parent->mnt_root == mnt->mnt_mountpoint) {
+ /* Got root (mnt)? */
+ if (mnt->mnt_parent == mnt)
+ break;
+ mnt = mnt->mnt_parent;
+ }
+ /*
+ * Clone all the read-only mounts and submounts, only if they
+ * are not shared or slave, and increment the hard read-only
+ * users count on each one. If this can't be done for every
+ * mount and submount below this one, fail.
+ */
+ cloned_tree = copy_tree(mnt, mnt->mnt_root,
+ CL_COPY_ALL | CL_PRIVATE |
+ CL_NO_SHARED | CL_NO_SLAVE |
+ CL_MAKE_HARD_READONLY);
+ if (IS_ERR(cloned_tree))
+ return PTR_ERR(cloned_tree);
+
+ topmost->mnt_sb->s_union_lower_mnts = cloned_tree;
+ return 0;
+}
+
/*
* @source_mnt : mount tree to be attached
* @nd : place the mount tree @source_mnt is attached
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 4014ff1..5643835 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -138,4 +138,6 @@ extern void mark_mounts_for_expiry(struct list_head *mounts);

extern dev_t name_to_dev_t(char *name);

+extern void put_union_sb(struct super_block *sb);
+
#endif /* _LINUX_MOUNT_H */
--
1.6.3.3


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