fs: btrfs: Crossport open_ctree_fs_info() from btrfs-progs
authorQu Wenruo <wqu@suse.com>
Wed, 24 Jun 2020 16:03:01 +0000 (18:03 +0200)
committerTom Rini <trini@konsulko.com>
Tue, 8 Sep 2020 00:57:27 +0000 (20:57 -0400)
open_ctree_fs_info() is the main entry point to open btrfs.

This version is a simplfied version of __open_ctree_fd() of btrfs-progs,
the main differences are:
- Parameters on how to specify a block device
  Instead of @fd and @path, U-Boot uses blk_desc and disk_partition_t.

- Remove open_ctree flags
  There won't be multiple open ctree modes in U-Boot.

Otherwise functions structures are all kept the same.

With open_ctree_fs_info() implemented, also introduce the global
current_fs_info pointer to show the current opened btrfs.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek BehĂșn <marek.behun@nic.cz>
fs/btrfs/Makefile
fs/btrfs/btrfs.c
fs/btrfs/btrfs.h
fs/btrfs/compat.h
fs/btrfs/ctree.h
fs/btrfs/disk-io.c
fs/btrfs/disk-io.h
fs/btrfs/root-tree.c [new file with mode: 0644]

index ec30aae..8d76422 100644 (file)
@@ -4,4 +4,4 @@
 
 obj-y := btrfs.o chunk-map.o compression.o ctree.o dev.o dir-item.o \
        extent-io.o inode.o root.o subvolume.o crypto/hash.o disk-io.o \
-       common/rbtree-utils.o extent-cache.o volumes.o
+       common/rbtree-utils.o extent-cache.o volumes.o root-tree.o
index f5266f1..2bd2ec9 100644 (file)
@@ -14,6 +14,7 @@
 #include "disk-io.h"
 
 struct btrfs_info btrfs_info;
+struct btrfs_fs_info *current_fs_info;
 
 static int readdir_callback(const struct __btrfs_root *root,
                            struct btrfs_dir_item *item)
@@ -81,6 +82,9 @@ static int readdir_callback(const struct __btrfs_root *root,
 int btrfs_probe(struct blk_desc *fs_dev_desc,
                struct disk_partition *fs_partition)
 {
+       struct btrfs_fs_info *fs_info;
+       int ret = -1;
+
        btrfs_blk_desc = fs_dev_desc;
        btrfs_part_info = fs_partition;
 
@@ -111,7 +115,12 @@ int btrfs_probe(struct blk_desc *fs_dev_desc,
                return -1;
        }
 
-       return 0;
+       fs_info = open_ctree_fs_info(fs_dev_desc, fs_partition);
+       if (fs_info) {
+               current_fs_info = fs_info;
+               ret = 0;
+       }
+       return ret;
 }
 
 int btrfs_ls(const char *path)
@@ -215,6 +224,10 @@ int btrfs_read(const char *file, void *buf, loff_t offset, loff_t len,
 void btrfs_close(void)
 {
        btrfs_chunk_map_exit();
+       if (current_fs_info) {
+               close_ctree_fs_info(current_fs_info);
+               current_fs_info = NULL;
+       }
 }
 
 int btrfs_uuid(char *uuid_str)
index 7dfdaf7..a52ff94 100644 (file)
@@ -22,6 +22,7 @@ struct btrfs_info {
 };
 
 extern struct btrfs_info btrfs_info;
+extern struct btrfs_fs_info *current_fs_info;
 
 /* dev.c */
 extern struct blk_desc *btrfs_blk_desc;
index be4f4e7..52de9db 100644 (file)
@@ -12,6 +12,8 @@
 /* A simple wraper to for error() used in btrfs-progs */
 #define error(fmt, ...)                pr_err("BTRFS: " fmt "\n", ##__VA_ARGS__)
 
+#define ASSERT(c) assert(c)
+
 #define BTRFS_UUID_UNPARSED_SIZE       37
 
 /*
index 7c814fd..b050b58 100644 (file)
@@ -1275,6 +1275,10 @@ const char *btrfs_super_csum_name(u16 csum_type);
 u16 btrfs_csum_type_size(u16 csum_type);
 size_t btrfs_super_num_csums(void);
 
+/* root-tree.c */
+int btrfs_find_last_root(struct btrfs_root *root, u64 objectid,
+                       struct btrfs_root_item *item, struct btrfs_key *key);
+
 /* ctree.c */
 int btrfs_comp_cpu_keys(const struct btrfs_key *k1, const struct btrfs_key *k2);
 enum btrfs_tree_block_status
index 65b4020..4d08b02 100644 (file)
@@ -4,6 +4,7 @@
 #include <uuid.h>
 #include <memalign.h>
 #include "kernel-shared/btrfs_tree.h"
+#include "common/rbtree-utils.h"
 #include "disk-io.h"
 #include "ctree.h"
 #include "btrfs.h"
@@ -424,24 +425,6 @@ out:
 
 }
 
-int btrfs_buffer_uptodate(struct extent_buffer *buf, u64 parent_transid)
-{
-       int ret;
-
-       ret = extent_buffer_uptodate(buf);
-       if (!ret)
-               return ret;
-
-       ret = verify_parent_transid(&buf->fs_info->extent_cache, buf,
-                                   parent_transid, 1);
-       return !ret;
-}
-
-int btrfs_set_buffer_uptodate(struct extent_buffer *eb)
-{
-       return set_extent_buffer_uptodate(eb);
-}
-
 int read_whole_eb(struct btrfs_fs_info *info, struct extent_buffer *eb, int mirror)
 {
        unsigned long offset = 0;
@@ -581,3 +564,493 @@ struct extent_buffer* read_tree_block(struct btrfs_fs_info *fs_info, u64 bytenr,
        free_extent_buffer(eb);
        return ERR_PTR(ret);
 }
+
+void btrfs_setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
+                     u64 objectid)
+{
+       root->node = NULL;
+       root->track_dirty = 0;
+
+       root->fs_info = fs_info;
+       root->objectid = objectid;
+       root->last_trans = 0;
+       root->last_inode_alloc = 0;
+
+       memset(&root->root_key, 0, sizeof(root->root_key));
+       memset(&root->root_item, 0, sizeof(root->root_item));
+       root->root_key.objectid = objectid;
+}
+
+static int find_and_setup_root(struct btrfs_root *tree_root,
+                              struct btrfs_fs_info *fs_info,
+                              u64 objectid, struct btrfs_root *root)
+{
+       int ret;
+       u64 generation;
+
+       btrfs_setup_root(root, fs_info, objectid);
+       ret = btrfs_find_last_root(tree_root, objectid,
+                                  &root->root_item, &root->root_key);
+       if (ret)
+               return ret;
+
+       generation = btrfs_root_generation(&root->root_item);
+       root->node = read_tree_block(fs_info,
+                       btrfs_root_bytenr(&root->root_item), generation);
+       if (!extent_buffer_uptodate(root->node))
+               return -EIO;
+
+       return 0;
+}
+
+int btrfs_free_fs_root(struct btrfs_root *root)
+{
+       if (root->node)
+               free_extent_buffer(root->node);
+       kfree(root);
+       return 0;
+}
+
+static void __free_fs_root(struct rb_node *node)
+{
+       struct btrfs_root *root;
+
+       root = container_of(node, struct btrfs_root, rb_node);
+       btrfs_free_fs_root(root);
+}
+
+FREE_RB_BASED_TREE(fs_roots, __free_fs_root);
+
+struct btrfs_root *btrfs_read_fs_root_no_cache(struct btrfs_fs_info *fs_info,
+                                              struct btrfs_key *location)
+{
+       struct btrfs_root *root;
+       struct btrfs_root *tree_root = fs_info->tree_root;
+       struct btrfs_path *path;
+       struct extent_buffer *l;
+       u64 generation;
+       int ret = 0;
+
+       root = calloc(1, sizeof(*root));
+       if (!root)
+               return ERR_PTR(-ENOMEM);
+       if (location->offset == (u64)-1) {
+               ret = find_and_setup_root(tree_root, fs_info,
+                                         location->objectid, root);
+               if (ret) {
+                       free(root);
+                       return ERR_PTR(ret);
+               }
+               goto insert;
+       }
+
+       btrfs_setup_root(root, fs_info,
+                        location->objectid);
+
+       path = btrfs_alloc_path();
+       if (!path) {
+               free(root);
+               return ERR_PTR(-ENOMEM);
+       }
+
+       ret = btrfs_search_slot(NULL, tree_root, location, path, 0, 0);
+       if (ret != 0) {
+               if (ret > 0)
+                       ret = -ENOENT;
+               goto out;
+       }
+       l = path->nodes[0];
+       read_extent_buffer(l, &root->root_item,
+              btrfs_item_ptr_offset(l, path->slots[0]),
+              sizeof(root->root_item));
+       memcpy(&root->root_key, location, sizeof(*location));
+
+       /* If this root is already an orphan, no need to read */
+       if (btrfs_root_refs(&root->root_item) == 0) {
+               ret = -ENOENT;
+               goto out;
+       }
+       ret = 0;
+out:
+       btrfs_free_path(path);
+       if (ret) {
+               free(root);
+               return ERR_PTR(ret);
+       }
+       generation = btrfs_root_generation(&root->root_item);
+       root->node = read_tree_block(fs_info,
+                       btrfs_root_bytenr(&root->root_item), generation);
+       if (!extent_buffer_uptodate(root->node)) {
+               free(root);
+               return ERR_PTR(-EIO);
+       }
+insert:
+       root->ref_cows = 1;
+       return root;
+}
+
+static int btrfs_fs_roots_compare_objectids(struct rb_node *node,
+                                           void *data)
+{
+       u64 objectid = *((u64 *)data);
+       struct btrfs_root *root;
+
+       root = rb_entry(node, struct btrfs_root, rb_node);
+       if (objectid > root->objectid)
+               return 1;
+       else if (objectid < root->objectid)
+               return -1;
+       else
+               return 0;
+}
+
+int btrfs_fs_roots_compare_roots(struct rb_node *node1, struct rb_node *node2)
+{
+       struct btrfs_root *root;
+
+       root = rb_entry(node2, struct btrfs_root, rb_node);
+       return btrfs_fs_roots_compare_objectids(node1, (void *)&root->objectid);
+}
+
+struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
+                                     struct btrfs_key *location)
+{
+       struct btrfs_root *root;
+       struct rb_node *node;
+       int ret;
+       u64 objectid = location->objectid;
+
+       if (location->objectid == BTRFS_ROOT_TREE_OBJECTID)
+               return fs_info->tree_root;
+       if (location->objectid == BTRFS_CHUNK_TREE_OBJECTID)
+               return fs_info->chunk_root;
+       if (location->objectid == BTRFS_CSUM_TREE_OBJECTID)
+               return fs_info->csum_root;
+       BUG_ON(location->objectid == BTRFS_TREE_RELOC_OBJECTID ||
+              location->offset != (u64)-1);
+
+       node = rb_search(&fs_info->fs_root_tree, (void *)&objectid,
+                        btrfs_fs_roots_compare_objectids, NULL);
+       if (node)
+               return container_of(node, struct btrfs_root, rb_node);
+
+       root = btrfs_read_fs_root_no_cache(fs_info, location);
+       if (IS_ERR(root))
+               return root;
+
+       ret = rb_insert(&fs_info->fs_root_tree, &root->rb_node,
+                       btrfs_fs_roots_compare_roots);
+       BUG_ON(ret);
+       return root;
+}
+
+void btrfs_free_fs_info(struct btrfs_fs_info *fs_info)
+{
+       free(fs_info->tree_root);
+       free(fs_info->chunk_root);
+       free(fs_info->csum_root);
+       free(fs_info->super_copy);
+       free(fs_info);
+}
+
+struct btrfs_fs_info *btrfs_new_fs_info(void)
+{
+       struct btrfs_fs_info *fs_info;
+
+       fs_info = calloc(1, sizeof(struct btrfs_fs_info));
+       if (!fs_info)
+               return NULL;
+
+       fs_info->tree_root = calloc(1, sizeof(struct btrfs_root));
+       fs_info->chunk_root = calloc(1, sizeof(struct btrfs_root));
+       fs_info->csum_root = calloc(1, sizeof(struct btrfs_root));
+       fs_info->super_copy = calloc(1, BTRFS_SUPER_INFO_SIZE);
+
+       if (!fs_info->tree_root || !fs_info->chunk_root ||
+           !fs_info->csum_root || !fs_info->super_copy)
+               goto free_all;
+
+       extent_io_tree_init(&fs_info->extent_cache);
+
+       fs_info->fs_root_tree = RB_ROOT;
+       cache_tree_init(&fs_info->mapping_tree.cache_tree);
+
+       mutex_init(&fs_info->fs_mutex);
+
+       return fs_info;
+free_all:
+       btrfs_free_fs_info(fs_info);
+       return NULL;
+}
+
+static int setup_root_or_create_block(struct btrfs_fs_info *fs_info,
+                                     struct btrfs_root *info_root,
+                                     u64 objectid, char *str)
+{
+       struct btrfs_root *root = fs_info->tree_root;
+       int ret;
+
+       ret = find_and_setup_root(root, fs_info, objectid, info_root);
+       if (ret) {
+               error("could not setup %s tree", str);
+               return -EIO;
+       }
+
+       return 0;
+}
+
+int btrfs_setup_all_roots(struct btrfs_fs_info *fs_info)
+{
+       struct btrfs_super_block *sb = fs_info->super_copy;
+       struct btrfs_root *root;
+       struct btrfs_key key;
+       u64 root_tree_bytenr;
+       u64 generation;
+       int ret;
+
+       root = fs_info->tree_root;
+       btrfs_setup_root(root, fs_info, BTRFS_ROOT_TREE_OBJECTID);
+       generation = btrfs_super_generation(sb);
+
+       root_tree_bytenr = btrfs_super_root(sb);
+
+       root->node = read_tree_block(fs_info, root_tree_bytenr, generation);
+       if (!extent_buffer_uptodate(root->node)) {
+               fprintf(stderr, "Couldn't read tree root\n");
+               return -EIO;
+       }
+
+       ret = setup_root_or_create_block(fs_info, fs_info->csum_root,
+                                        BTRFS_CSUM_TREE_OBJECTID, "csum");
+       if (ret)
+               return ret;
+       fs_info->csum_root->track_dirty = 1;
+
+       fs_info->last_trans_committed = generation;
+
+       key.objectid = BTRFS_FS_TREE_OBJECTID;
+       key.type = BTRFS_ROOT_ITEM_KEY;
+       key.offset = (u64)-1;
+       fs_info->fs_root = btrfs_read_fs_root(fs_info, &key);
+
+       if (IS_ERR(fs_info->fs_root))
+               return -EIO;
+       return 0;
+}
+
+void btrfs_release_all_roots(struct btrfs_fs_info *fs_info)
+{
+       if (fs_info->csum_root)
+               free_extent_buffer(fs_info->csum_root->node);
+       if (fs_info->tree_root)
+               free_extent_buffer(fs_info->tree_root->node);
+       if (fs_info->chunk_root)
+               free_extent_buffer(fs_info->chunk_root->node);
+}
+
+static void free_map_lookup(struct cache_extent *ce)
+{
+       struct map_lookup *map;
+
+       map = container_of(ce, struct map_lookup, ce);
+       kfree(map);
+}
+
+FREE_EXTENT_CACHE_BASED_TREE(mapping_cache, free_map_lookup);
+
+void btrfs_cleanup_all_caches(struct btrfs_fs_info *fs_info)
+{
+       free_mapping_cache_tree(&fs_info->mapping_tree.cache_tree);
+       extent_io_tree_cleanup(&fs_info->extent_cache);
+}
+
+static int btrfs_scan_fs_devices(struct blk_desc *desc,
+                                struct disk_partition *part,
+                                struct btrfs_fs_devices **fs_devices)
+{
+       u64 total_devs;
+       int ret;
+
+       if (round_up(BTRFS_SUPER_INFO_SIZE + BTRFS_SUPER_INFO_OFFSET,
+                    desc->blksz) > (part->size << desc->log2blksz)) {
+               error("superblock end %u is larger than device size " LBAFU,
+                               BTRFS_SUPER_INFO_SIZE + BTRFS_SUPER_INFO_OFFSET,
+                               part->size << desc->log2blksz);
+               return -EINVAL;
+       }
+
+       ret = btrfs_scan_one_device(desc, part, fs_devices, &total_devs);
+       if (ret) {
+               fprintf(stderr, "No valid Btrfs found\n");
+               return ret;
+       }
+       return 0;
+}
+
+int btrfs_check_fs_compatibility(struct btrfs_super_block *sb)
+{
+       u64 features;
+
+       features = btrfs_super_incompat_flags(sb) &
+                  ~BTRFS_FEATURE_INCOMPAT_SUPP;
+       if (features) {
+               printk("couldn't open because of unsupported "
+                      "option features (%llx).\n",
+                      (unsigned long long)features);
+               return -ENOTSUPP;
+       }
+
+       features = btrfs_super_incompat_flags(sb);
+       if (!(features & BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF)) {
+               features |= BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF;
+               btrfs_set_super_incompat_flags(sb, features);
+       }
+
+       return 0;
+}
+
+static int btrfs_setup_chunk_tree_and_device_map(struct btrfs_fs_info *fs_info)
+{
+       struct btrfs_super_block *sb = fs_info->super_copy;
+       u64 chunk_root_bytenr;
+       u64 generation;
+       int ret;
+
+       btrfs_setup_root(fs_info->chunk_root, fs_info,
+                       BTRFS_CHUNK_TREE_OBJECTID);
+
+       ret = btrfs_read_sys_array(fs_info);
+       if (ret)
+               return ret;
+
+       generation = btrfs_super_chunk_root_generation(sb);
+       chunk_root_bytenr = btrfs_super_chunk_root(sb);
+
+       fs_info->chunk_root->node = read_tree_block(fs_info,
+                                                   chunk_root_bytenr,
+                                                   generation);
+       if (!extent_buffer_uptodate(fs_info->chunk_root->node)) {
+               error("cannot read chunk root");
+               return -EIO;
+       }
+
+       ret = btrfs_read_chunk_tree(fs_info);
+       if (ret) {
+               fprintf(stderr, "Couldn't read chunk tree\n");
+               return ret;
+       }
+       return 0;
+}
+
+struct btrfs_fs_info *open_ctree_fs_info(struct blk_desc *desc,
+                                        struct disk_partition *part)
+{
+       struct btrfs_fs_info *fs_info;
+       struct btrfs_super_block *disk_super;
+       struct btrfs_fs_devices *fs_devices = NULL;
+       struct extent_buffer *eb;
+       int ret;
+
+       fs_info = btrfs_new_fs_info();
+       if (!fs_info) {
+               fprintf(stderr, "Failed to allocate memory for fs_info\n");
+               return NULL;
+       }
+
+       ret = btrfs_scan_fs_devices(desc, part, &fs_devices);
+       if (ret)
+               goto out;
+
+       fs_info->fs_devices = fs_devices;
+
+       ret = btrfs_open_devices(fs_devices);
+       if (ret)
+               goto out;
+
+       disk_super = fs_info->super_copy;
+       ret = btrfs_read_dev_super(desc, part, disk_super);
+       if (ret) {
+               printk("No valid btrfs found\n");
+               goto out_devices;
+       }
+
+       if (btrfs_super_flags(disk_super) & BTRFS_SUPER_FLAG_CHANGING_FSID) {
+               fprintf(stderr, "ERROR: Filesystem UUID change in progress\n");
+               goto out_devices;
+       }
+
+       ASSERT(!memcmp(disk_super->fsid, fs_devices->fsid, BTRFS_FSID_SIZE));
+       if (btrfs_fs_incompat(fs_info, METADATA_UUID))
+               ASSERT(!memcmp(disk_super->metadata_uuid,
+                              fs_devices->metadata_uuid, BTRFS_FSID_SIZE));
+
+       fs_info->sectorsize = btrfs_super_sectorsize(disk_super);
+       fs_info->nodesize = btrfs_super_nodesize(disk_super);
+       fs_info->stripesize = btrfs_super_stripesize(disk_super);
+
+       ret = btrfs_check_fs_compatibility(fs_info->super_copy);
+       if (ret)
+               goto out_devices;
+
+       ret = btrfs_setup_chunk_tree_and_device_map(fs_info);
+       if (ret)
+               goto out_chunk;
+
+       /* Chunk tree root is unable to read, return directly */
+       if (!fs_info->chunk_root)
+               return fs_info;
+
+       eb = fs_info->chunk_root->node;
+       read_extent_buffer(eb, fs_info->chunk_tree_uuid,
+                          btrfs_header_chunk_tree_uuid(eb),
+                          BTRFS_UUID_SIZE);
+
+       ret = btrfs_setup_all_roots(fs_info);
+       if (ret)
+               goto out_chunk;
+
+       return fs_info;
+
+out_chunk:
+       btrfs_release_all_roots(fs_info);
+       btrfs_cleanup_all_caches(fs_info);
+out_devices:
+       btrfs_close_devices(fs_devices);
+out:
+       btrfs_free_fs_info(fs_info);
+       return NULL;
+}
+
+int close_ctree_fs_info(struct btrfs_fs_info *fs_info)
+{
+       int ret;
+       int err = 0;
+
+       free_fs_roots_tree(&fs_info->fs_root_tree);
+
+       btrfs_release_all_roots(fs_info);
+       ret = btrfs_close_devices(fs_info->fs_devices);
+       btrfs_cleanup_all_caches(fs_info);
+       btrfs_free_fs_info(fs_info);
+       if (!err)
+               err = ret;
+       return err;
+}
+
+int btrfs_buffer_uptodate(struct extent_buffer *buf, u64 parent_transid)
+{
+       int ret;
+
+       ret = extent_buffer_uptodate(buf);
+       if (!ret)
+               return ret;
+
+       ret = verify_parent_transid(&buf->fs_info->extent_cache, buf,
+                                   parent_transid, 1);
+       return !ret;
+}
+
+int btrfs_set_buffer_uptodate(struct extent_buffer *eb)
+{
+       return set_extent_buffer_uptodate(eb);
+}
index ca731a5..9daf959 100644 (file)
@@ -22,6 +22,25 @@ struct extent_buffer* btrfs_find_create_tree_block(
                struct btrfs_fs_info *fs_info, u64 bytenr);
 struct extent_buffer *btrfs_find_tree_block(struct btrfs_fs_info *fs_info,
                                            u64 bytenr, u32 blocksize);
+struct btrfs_root *btrfs_read_fs_root_no_cache(struct btrfs_fs_info *fs_info,
+                                              struct btrfs_key *location);
+struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
+                                     struct btrfs_key *location);
+
+void btrfs_setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
+                     u64 objectid);
+
+void btrfs_free_fs_info(struct btrfs_fs_info *fs_info);
+struct btrfs_fs_info *btrfs_new_fs_info(void);
+int btrfs_check_fs_compatibility(struct btrfs_super_block *sb);
+int btrfs_setup_all_roots(struct btrfs_fs_info *fs_info);
+void btrfs_release_all_roots(struct btrfs_fs_info *fs_info);
+void btrfs_cleanup_all_caches(struct btrfs_fs_info *fs_info);
+
+struct btrfs_fs_info *open_ctree_fs_info(struct blk_desc *desc,
+                                        struct disk_partition *part);
+int close_ctree_fs_info(struct btrfs_fs_info *fs_info);
+
 int btrfs_read_dev_super(struct blk_desc *desc, struct disk_partition *part,
                         struct btrfs_super_block *sb);
 int btrfs_buffer_uptodate(struct extent_buffer *buf, u64 parent_transid);
diff --git a/fs/btrfs/root-tree.c b/fs/btrfs/root-tree.c
new file mode 100644 (file)
index 0000000..a39ad72
--- /dev/null
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include "ctree.h"
+
+int btrfs_find_last_root(struct btrfs_root *root, u64 objectid,
+                       struct btrfs_root_item *item, struct btrfs_key *key)
+{
+       struct btrfs_path *path;
+       struct btrfs_key search_key;
+       struct btrfs_key found_key;
+       struct extent_buffer *l;
+       int ret;
+       int slot;
+
+       path = btrfs_alloc_path();
+       if (!path)
+               return -ENOMEM;
+
+       search_key.objectid = objectid;
+       search_key.type = BTRFS_ROOT_ITEM_KEY;
+       search_key.offset = (u64)-1;
+
+       ret = btrfs_search_slot(NULL, root, &search_key, path, 0, 0);
+       if (ret < 0)
+               goto out;
+       if (path->slots[0] == 0) {
+               ret = -ENOENT;
+               goto out;
+       }
+
+       BUG_ON(ret == 0);
+       l = path->nodes[0];
+       slot = path->slots[0] - 1;
+       btrfs_item_key_to_cpu(l, &found_key, slot);
+       if (found_key.type != BTRFS_ROOT_ITEM_KEY ||
+           found_key.objectid != objectid) {
+               ret = -ENOENT;
+               goto out;
+       }
+       read_extent_buffer(l, item, btrfs_item_ptr_offset(l, slot),
+                          sizeof(*item));
+       memcpy(key, &found_key, sizeof(found_key));
+       ret = 0;
+out:
+       btrfs_free_path(path);
+       return ret;
+}