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Subject[PATCH v2 5/6] Btrfs: unify subvol= and subvolid= mounting
Date
Currently, mounting a subvolume with subvolid= takes a different code
path than mounting with subvol=. This isn't really a big deal except for
the fact that mounts done with subvolid= or the default subvolume don't
have a dentry that's connected to the dentry tree like in the subvol=
case. To unify the code paths, when given subvolid= or using the default
subvolume ID, translate it into a subvolume name by walking
ROOT_BACKREFs in the root tree and INODE_REFs in the filesystem trees.

Signed-off-by: Omar Sandoval <osandov@osandov.com>
---
fs/btrfs/super.c | 229 +++++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 171 insertions(+), 58 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 20b470d..80a8047 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -841,33 +841,153 @@ out:
return error;
}

-static struct dentry *get_default_root(struct super_block *sb,
- u64 subvol_objectid)
+static char *get_subvol_name_from_objectid(struct btrfs_fs_info *fs_info,
+ u64 subvol_objectid)
{
- struct btrfs_fs_info *fs_info = btrfs_sb(sb);
struct btrfs_root *root = fs_info->tree_root;
- struct btrfs_root *new_root;
- struct btrfs_dir_item *di;
- struct btrfs_path *path;
- struct btrfs_key location;
- struct inode *inode;
- u64 dir_id;
- int new = 0;
+ struct btrfs_root *fs_root;
+ struct btrfs_root_ref *root_ref;
+ struct btrfs_inode_ref *inode_ref;
+ struct btrfs_key key;
+ struct btrfs_path *path = NULL;
+ char *name = NULL, *ptr;
+ u64 dirid;
+ int len;
+ int ret;
+
+ path = btrfs_alloc_path();
+ if (!path) {
+ ret = -ENOMEM;
+ goto err;
+ }
+ path->leave_spinning = 1;
+
+ name = kmalloc(PATH_MAX, GFP_NOFS);
+ if (!name) {
+ ret = -ENOMEM;
+ goto err;
+ }
+ ptr = name + PATH_MAX - 1;
+ ptr[0] = '\0';

/*
- * We have a specific subvol we want to mount, just setup location and
- * go look up the root.
+ * Walk up the subvolume trees in the tree of tree roots by root
+ * backrefs until we hit the top-level subvolume.
*/
- if (subvol_objectid) {
- location.objectid = subvol_objectid;
- location.type = BTRFS_ROOT_ITEM_KEY;
- location.offset = (u64)-1;
- goto find_root;
+ while (subvol_objectid != BTRFS_FS_TREE_OBJECTID) {
+ key.objectid = subvol_objectid;
+ key.type = BTRFS_ROOT_BACKREF_KEY;
+ key.offset = (u64)-1;
+
+ ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+ if (ret < 0) {
+ goto err;
+ } else if (ret > 0) {
+ ret = btrfs_previous_item(root, path, subvol_objectid,
+ BTRFS_ROOT_BACKREF_KEY);
+ if (ret < 0) {
+ goto err;
+ } else if (ret > 0) {
+ ret = -ENOENT;
+ goto err;
+ }
+ }
+
+ btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+ subvol_objectid = key.offset;
+
+ root_ref = btrfs_item_ptr(path->nodes[0], path->slots[0],
+ struct btrfs_root_ref);
+ len = btrfs_root_ref_name_len(path->nodes[0], root_ref);
+ ptr -= len + 1;
+ if (ptr < name) {
+ ret = -ENAMETOOLONG;
+ goto err;
+ }
+ read_extent_buffer(path->nodes[0], ptr + 1,
+ (unsigned long)(root_ref + 1), len);
+ ptr[0] = '/';
+ dirid = btrfs_root_ref_dirid(path->nodes[0], root_ref);
+ btrfs_release_path(path);
+
+ key.objectid = subvol_objectid;
+ key.type = BTRFS_ROOT_ITEM_KEY;
+ key.offset = (u64)-1;
+ fs_root = btrfs_read_fs_root_no_name(fs_info, &key);
+ if (IS_ERR(fs_root)) {
+ ret = PTR_ERR(fs_root);
+ goto err;
+ }
+
+ /*
+ * Walk up the filesystem tree by inode refs until we hit the
+ * root directory.
+ */
+ while (dirid != BTRFS_FIRST_FREE_OBJECTID) {
+ key.objectid = dirid;
+ key.type = BTRFS_INODE_REF_KEY;
+ key.offset = (u64)-1;
+
+ ret = btrfs_search_slot(NULL, fs_root, &key, path, 0, 0);
+ if (ret < 0) {
+ goto err;
+ } else if (ret > 0) {
+ ret = btrfs_previous_item(fs_root, path, dirid,
+ BTRFS_INODE_REF_KEY);
+ if (ret < 0) {
+ goto err;
+ } else if (ret > 0) {
+ ret = -ENOENT;
+ goto err;
+ }
+ }
+
+ btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+ dirid = key.offset;
+
+ inode_ref = btrfs_item_ptr(path->nodes[0],
+ path->slots[0],
+ struct btrfs_inode_ref);
+ len = btrfs_inode_ref_name_len(path->nodes[0],
+ inode_ref);
+ ptr -= len + 1;
+ if (ptr < name) {
+ ret = -ENAMETOOLONG;
+ goto err;
+ }
+ read_extent_buffer(path->nodes[0], ptr + 1,
+ (unsigned long)(inode_ref + 1), len);
+ ptr[0] = '/';
+ btrfs_release_path(path);
+ }
}

+ btrfs_free_path(path);
+ if (ptr == name + PATH_MAX - 1) {
+ name[0] = '/';
+ name[1] = '\0';
+ } else {
+ memmove(name, ptr, name + PATH_MAX - ptr);
+ }
+ return name;
+
+err:
+ btrfs_free_path(path);
+ kfree(name);
+ return ERR_PTR(ret);
+}
+
+static int get_default_subvol_objectid(struct btrfs_fs_info *fs_info, u64 *objectid)
+{
+ struct btrfs_root *root = fs_info->tree_root;
+ struct btrfs_dir_item *di;
+ struct btrfs_path *path;
+ struct btrfs_key location;
+ u64 dir_id;
+
path = btrfs_alloc_path();
if (!path)
- return ERR_PTR(-ENOMEM);
+ return -ENOMEM;
path->leave_spinning = 1;

/*
@@ -879,49 +999,23 @@ static struct dentry *get_default_root(struct super_block *sb,
di = btrfs_lookup_dir_item(NULL, root, path, dir_id, "default", 7, 0);
if (IS_ERR(di)) {
btrfs_free_path(path);
- return ERR_CAST(di);
+ return PTR_ERR(di);
}
if (!di) {
/*
* Ok the default dir item isn't there. This is weird since
* it's always been there, but don't freak out, just try and
- * mount to root most subvolume.
+ * mount the top-level subvolume.
*/
btrfs_free_path(path);
- dir_id = BTRFS_FIRST_FREE_OBJECTID;
- new_root = fs_info->fs_root;
- goto setup_root;
+ *objectid = BTRFS_FS_TREE_OBJECTID;
+ return 0;
}

btrfs_dir_item_key_to_cpu(path->nodes[0], di, &location);
btrfs_free_path(path);
-
-find_root:
- new_root = btrfs_read_fs_root_no_name(fs_info, &location);
- if (IS_ERR(new_root))
- return ERR_CAST(new_root);
-
- dir_id = btrfs_root_dirid(&new_root->root_item);
-setup_root:
- location.objectid = dir_id;
- location.type = BTRFS_INODE_ITEM_KEY;
- location.offset = 0;
-
- inode = btrfs_iget(sb, &location, new_root, &new);
- if (IS_ERR(inode))
- return ERR_CAST(inode);
-
- /*
- * If we're just mounting the root most subvol put the inode and return
- * a reference to the dentry. We will have already gotten a reference
- * to the inode in btrfs_fill_super so we're good to go.
- */
- if (!new && sb->s_root->d_inode == inode) {
- iput(inode);
- return dget(sb->s_root);
- }
-
- return d_obtain_root(inode);
+ *objectid = location.objectid;
+ return 0;
}

static int btrfs_fill_super(struct super_block *sb,
@@ -1207,6 +1301,25 @@ static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
goto out;
}

+ if (!subvol_name) {
+ if (!subvol_objectid) {
+ ret = get_default_subvol_objectid(btrfs_sb(mnt->mnt_sb),
+ &subvol_objectid);
+ if (ret) {
+ root = ERR_PTR(ret);
+ goto out;
+ }
+ }
+ subvol_name = get_subvol_name_from_objectid(btrfs_sb(mnt->mnt_sb),
+ subvol_objectid);
+ if (IS_ERR(subvol_name)) {
+ root = ERR_CAST(subvol_name);
+ subvol_name = NULL;
+ goto out;
+ }
+
+ }
+
root = mount_subtree(mnt, subvol_name);
/* mount_subtree() drops our reference on the vfsmount. */
mnt = NULL;
@@ -1222,6 +1335,11 @@ static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
ret = -EINVAL;
}
if (subvol_objectid && root_objectid != subvol_objectid) {
+ /*
+ * This will also catch a race condition where a
+ * subvolume which was passed by ID is renamed and
+ * another subvolume is renamed over the old location.
+ */
pr_err("BTRFS: subvol '%s' does not match subvolid %llu\n",
subvol_name, subvol_objectid);
ret = -EINVAL;
@@ -1301,7 +1419,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
{
struct block_device *bdev = NULL;
struct super_block *s;
- struct dentry *root;
struct btrfs_fs_devices *fs_devices = NULL;
struct btrfs_fs_info *fs_info = NULL;
struct security_mnt_opts new_sec_opts;
@@ -1321,7 +1438,7 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
return ERR_PTR(error);
}

- if (subvol_name) {
+ if (subvol_name || subvol_objectid != BTRFS_FS_TREE_OBJECTID) {
/* mount_subvol() will free subvol_name. */
return mount_subvol(subvol_name, subvol_objectid, flags,
device_name, data);
@@ -1390,23 +1507,19 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
error = btrfs_fill_super(s, fs_devices, data,
flags & MS_SILENT ? 1 : 0);
}
-
- root = !error ? get_default_root(s, subvol_objectid) : ERR_PTR(error);
- if (IS_ERR(root)) {
+ if (error) {
deactivate_locked_super(s);
- error = PTR_ERR(root);
goto error_sec_opts;
}

fs_info = btrfs_sb(s);
error = setup_security_options(fs_info, s, &new_sec_opts);
if (error) {
- dput(root);
deactivate_locked_super(s);
goto error_sec_opts;
}

- return root;
+ return dget(s->s_root);

error_close_devices:
btrfs_close_devices(fs_devices);
--
2.3.5


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