/*
 * Copyright (C) 2011 STRATO.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include "ctree.h"
#include "disk-io.h"
#include "backref.h"
#include "ulist.h"
#include "transaction.h"
#include "delayed-ref.h"
#include "locking.h"

struct extent_inode_elem {
        u64 inum;
        u64 offset;
        struct extent_inode_elem *next;
};

static int check_extent_in_eb(struct btrfs_key *key, struct extent_buffer *eb,
                                struct btrfs_file_extent_item *fi,
                                u64 extent_item_pos,
                                struct extent_inode_elem **eie)
{
        u64 data_offset;
        u64 data_len;
        struct extent_inode_elem *e;

        data_offset = btrfs_file_extent_offset(eb, fi);
        data_len = btrfs_file_extent_num_bytes(eb, fi);

        if (extent_item_pos < data_offset ||
            extent_item_pos >= data_offset + data_len)
                return 1;

        e = kmalloc(sizeof(*e), GFP_NOFS);
        if (!e)
                return -ENOMEM;

        e->next = *eie;
        e->inum = key->objectid;
        e->offset = key->offset + (extent_item_pos - data_offset);
        *eie = e;

        return 0;
}

static int find_extent_in_eb(struct extent_buffer *eb, u64 wanted_disk_byte,
                                u64 extent_item_pos,
                                struct extent_inode_elem **eie)
{
        u64 disk_byte;
        struct btrfs_key key;
        struct btrfs_file_extent_item *fi;
        int slot;
        int nritems;
        int extent_type;
        int ret;

        /*
         * from the shared data ref, we only have the leaf but we need
         * the key. thus, we must look into all items and see that we
         * find one (some) with a reference to our extent item.
         */
        nritems = btrfs_header_nritems(eb);
        for (slot = 0; slot < nritems; ++slot) {
                btrfs_item_key_to_cpu(eb, &key, slot);
                if (key.type != BTRFS_EXTENT_DATA_KEY)
                        continue;
                fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
                extent_type = btrfs_file_extent_type(eb, fi);
                if (extent_type == BTRFS_FILE_EXTENT_INLINE)
                        continue;
                /* don't skip BTRFS_FILE_EXTENT_PREALLOC, we can handle that */
                disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);
                if (disk_byte != wanted_disk_byte)
                        continue;

                ret = check_extent_in_eb(&key, eb, fi, extent_item_pos, eie);
                if (ret < 0)
                        return ret;
        }

        return 0;
}

/*
 * this structure records all encountered refs on the way up to the root
 */
struct __prelim_ref {
        struct list_head list;
        u64 root_id;
        struct btrfs_key key_for_search;
        int level;
        int count;
        struct extent_inode_elem *inode_list;
        u64 parent;
        u64 wanted_disk_byte;
};

/*
 * the rules for all callers of this function are:
 * - obtaining the parent is the goal
 * - if you add a key, you must know that it is a correct key
 * - if you cannot add the parent or a correct key, then we will look into the
 *   block later to set a correct key
 *
 * delayed refs
 * ============
 *        backref type | shared | indirect | shared | indirect
 * information         |   tree |     tree |   data |     data
 * --------------------+--------+----------+--------+----------
 *      parent logical |    y   |     -    |    -   |     -
 *      key to resolve |    -   |     y    |    y   |     y
 *  tree block logical |    -   |     -    |    -   |     -
 *  root for resolving |    y   |     y    |    y   |     y
 *
 * - column 1:       we've the parent -> done
 * - column 2, 3, 4: we use the key to find the parent
 *
 * on disk refs (inline or keyed)
 * ==============================
 *        backref type | shared | indirect | shared | indirect
 * information         |   tree |     tree |   data |     data
 * --------------------+--------+----------+--------+----------
 *      parent logical |    y   |     -    |    y   |     -
 *      key to resolve |    -   |     -    |    -   |     y
 *  tree block logical |    y   |     y    |    y   |     y
 *  root for resolving |    -   |     y    |    y   |     y
 *
 * - column 1, 3: we've the parent -> done
 * - column 2:    we take the first key from the block to find the parent
 *                (see __add_missing_keys)
 * - column 4:    we use the key to find the parent
 *
 * additional information that's available but not required to find the parent
 * block might help in merging entries to gain some speed.
 */

static int __add_prelim_ref(struct list_head *head, u64 root_id,
                            struct btrfs_key *key, int level,
                            u64 parent, u64 wanted_disk_byte, int count)
{
        struct __prelim_ref *ref;

        /* in case we're adding delayed refs, we're holding the refs spinlock */
        ref = kmalloc(sizeof(*ref), GFP_ATOMIC);
        if (!ref)
                return -ENOMEM;

        ref->root_id = root_id;
        if (key)
                ref->key_for_search = *key;
        else
                memset(&ref->key_for_search, 0, sizeof(ref->key_for_search));

        ref->inode_list = NULL;
        ref->level = level;
        ref->count = count;
        ref->parent = parent;
        ref->wanted_disk_byte = wanted_disk_byte;
        list_add_tail(&ref->list, head);

        return 0;
}

static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
                                struct ulist *parents, int level,
                                struct btrfs_key *key_for_search, u64 time_seq,
                                u64 wanted_disk_byte,
                                const u64 *extent_item_pos)
{
        int ret = 0;
        int slot;
        struct extent_buffer *eb;
        struct btrfs_key key;
        struct btrfs_file_extent_item *fi;
        struct extent_inode_elem *eie = NULL;
        u64 disk_byte;

        if (level != 0) {
                eb = path->nodes[level];
                ret = ulist_add(parents, eb->start, 0, GFP_NOFS);
                if (ret < 0)
                        return ret;
                return 0;
        }

        /*
         * We normally enter this function with the path already pointing to
         * the first item to check. But sometimes, we may enter it with
         * slot==nritems. In that case, go to the next leaf before we continue.
         */
        if (path->slots[0] >= btrfs_header_nritems(path->nodes[0]))
                ret = btrfs_next_old_leaf(root, path, time_seq);

        while (!ret) {
                eb = path->nodes[0];
                slot = path->slots[0];

                btrfs_item_key_to_cpu(eb, &key, slot);

                if (key.objectid != key_for_search->objectid ||
                    key.type != BTRFS_EXTENT_DATA_KEY)
                        break;

                fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
                disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);

                if (disk_byte == wanted_disk_byte) {
                        eie = NULL;
                        if (extent_item_pos) {
                                ret = check_extent_in_eb(&key, eb, fi,
                                                *extent_item_pos,
                                                &eie);
                                if (ret < 0)
                                        break;
                        }
                        if (!ret) {
                                ret = ulist_add(parents, eb->start,
                                                (unsigned long)eie, GFP_NOFS);
                                if (ret < 0)
                                        break;
                                if (!extent_item_pos) {
                                        ret = btrfs_next_old_leaf(root, path,
                                                        time_seq);
                                        continue;
                                }
                        }
                }
                ret = btrfs_next_old_item(root, path, time_seq);
        }

        if (ret > 0)
                ret = 0;
        return ret;
}

/*
 * resolve an indirect backref in the form (root_id, key, level)
 * to a logical address
 */
static int __resolve_indirect_ref(struct btrfs_fs_info *fs_info,
                                        int search_commit_root,
                                        u64 time_seq,
                                        struct __prelim_ref *ref,
                                        struct ulist *parents,
                                        const u64 *extent_item_pos)
{
        struct btrfs_path *path;
        struct btrfs_root *root;
        struct btrfs_key root_key;
        struct extent_buffer *eb;
        int ret = 0;
        int root_level;
        int level = ref->level;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
        path->search_commit_root = !!search_commit_root;

        root_key.objectid = ref->root_id;
        root_key.type = BTRFS_ROOT_ITEM_KEY;
        root_key.offset = (u64)-1;
        root = btrfs_read_fs_root_no_name(fs_info, &root_key);
        if (IS_ERR(root)) {
                ret = PTR_ERR(root);
                goto out;
        }

        rcu_read_lock();
        root_level = btrfs_header_level(root->node);
        rcu_read_unlock();

        if (root_level + 1 == level)
                goto out;

        path->lowest_level = level;
        ret = btrfs_search_old_slot(root, &ref->key_for_search, path, time_seq);
        pr_debug("search slot in root %llu (level %d, ref count %d) returned "
                 "%d for key (%llu %u %llu)\n",
                 (unsigned long long)ref->root_id, level, ref->count, ret,
                 (unsigned long long)ref->key_for_search.objectid,
                 ref->key_for_search.type,
                 (unsigned long long)ref->key_for_search.offset);
        if (ret < 0)
                goto out;

        eb = path->nodes[level];
        while (!eb) {
                if (!level) {
                        WARN_ON(1);
                        ret = 1;
                        goto out;
                }
                level--;
                eb = path->nodes[level];
        }

        ret = add_all_parents(root, path, parents, level, &ref->key_for_search,
                                time_seq, ref->wanted_disk_byte,
                                extent_item_pos);
out:
        btrfs_free_path(path);
        return ret;
}

/*
 * resolve all indirect backrefs from the list
 */
static int __resolve_indirect_refs(struct btrfs_fs_info *fs_info,
                                   int search_commit_root, u64 time_seq,
                                   struct list_head *head,
                                   const u64 *extent_item_pos)
{
        int err;
        int ret = 0;
        struct __prelim_ref *ref;
        struct __prelim_ref *ref_safe;
        struct __prelim_ref *new_ref;
        struct ulist *parents;
        struct ulist_node *node;
        struct ulist_iterator uiter;

        parents = ulist_alloc(GFP_NOFS);
        if (!parents)
                return -ENOMEM;

        /*
         * _safe allows us to insert directly after the current item without
         * iterating over the newly inserted items.
         * we're also allowed to re-assign ref during iteration.
         */
        list_for_each_entry_safe(ref, ref_safe, head, list) {
                if (ref->parent)        /* already direct */
                        continue;
                if (ref->count == 0)
                        continue;
                err = __resolve_indirect_ref(fs_info, search_commit_root,
                                             time_seq, ref, parents,
                                             extent_item_pos);
                if (err) {
                        if (ret == 0)
                                ret = err;
                        continue;
                }

                /* we put the first parent into the ref at hand */
                ULIST_ITER_INIT(&uiter);
                node = ulist_next(parents, &uiter);
                ref->parent = node ? node->val : 0;
                ref->inode_list =
                        node ? (struct extent_inode_elem *)node->aux : 0;

                /* additional parents require new refs being added here */
                while ((node = ulist_next(parents, &uiter))) {
                        new_ref = kmalloc(sizeof(*new_ref), GFP_NOFS);
                        if (!new_ref) {
                                ret = -ENOMEM;
                                break;
                        }
                        memcpy(new_ref, ref, sizeof(*ref));
                        new_ref->parent = node->val;
                        new_ref->inode_list =
                                        (struct extent_inode_elem *)node->aux;
                        list_add(&new_ref->list, &ref->list);
                }
                ulist_reinit(parents);
        }

        ulist_free(parents);
        return ret;
}

static inline int ref_for_same_block(struct __prelim_ref *ref1,
                                     struct __prelim_ref *ref2)
{
        if (ref1->level != ref2->level)
                return 0;
        if (ref1->root_id != ref2->root_id)
                return 0;
        if (ref1->key_for_search.type != ref2->key_for_search.type)
                return 0;
        if (ref1->key_for_search.objectid != ref2->key_for_search.objectid)
                return 0;
        if (ref1->key_for_search.offset != ref2->key_for_search.offset)
                return 0;
        if (ref1->parent != ref2->parent)
                return 0;

        return 1;
}

/*
 * read tree blocks and add keys where required.
 */
static int __add_missing_keys(struct btrfs_fs_info *fs_info,
                              struct list_head *head)
{
        struct list_head *pos;
        struct extent_buffer *eb;

        list_for_each(pos, head) {
                struct __prelim_ref *ref;
                ref = list_entry(pos, struct __prelim_ref, list);

                if (ref->parent)
                        continue;
                if (ref->key_for_search.type)
                        continue;
                BUG_ON(!ref->wanted_disk_byte);
                eb = read_tree_block(fs_info->tree_root, ref->wanted_disk_byte,
                                     fs_info->tree_root->leafsize, 0);
                BUG_ON(!eb);
                btrfs_tree_read_lock(eb);
                if (btrfs_header_level(eb) == 0)
                        btrfs_item_key_to_cpu(eb, &ref->key_for_search, 0);
                else
                        btrfs_node_key_to_cpu(eb, &ref->key_for_search, 0);
                btrfs_tree_read_unlock(eb);
                free_extent_buffer(eb);
        }
        return 0;
}

/*
 * merge two lists of backrefs and adjust counts accordingly
 *
 * mode = 1: merge identical keys, if key is set
 *    FIXME: if we add more keys in __add_prelim_ref, we can merge more here.
 *           additionally, we could even add a key range for the blocks we
 *           looked into to merge even more (-> replace unresolved refs by those
 *           having a parent).
 * mode = 2: merge identical parents
 */
static int __merge_refs(struct list_head *head, int mode)
{
        struct list_head *pos1;

        list_for_each(pos1, head) {
                struct list_head *n2;
                struct list_head *pos2;
                struct __prelim_ref *ref1;

                ref1 = list_entry(pos1, struct __prelim_ref, list);

                for (pos2 = pos1->next, n2 = pos2->next; pos2 != head;
                     pos2 = n2, n2 = pos2->next) {
                        struct __prelim_ref *ref2;
                        struct __prelim_ref *xchg;

                        ref2 = list_entry(pos2, struct __prelim_ref, list);

                        if (mode == 1) {
                                if (!ref_for_same_block(ref1, ref2))
                                        continue;
                                if (!ref1->parent && ref2->parent) {
                                        xchg = ref1;
                                        ref1 = ref2;
                                        ref2 = xchg;
                                }
                                ref1->count += ref2->count;
                        } else {
                                if (ref1->parent != ref2->parent)
                                        continue;
                                ref1->count += ref2->count;
                        }
                        list_del(&ref2->list);
                        kfree(ref2);
                }

        }
        return 0;
}

/*
 * add all currently queued delayed refs from this head whose seq nr is
 * smaller or equal that seq to the list
 */
static int __add_delayed_refs(struct btrfs_delayed_ref_head *head, u64 seq,
                              struct list_head *prefs)
{
        struct btrfs_delayed_extent_op *extent_op = head->extent_op;
        struct rb_node *n = &head->node.rb_node;
        struct btrfs_key key;
        struct btrfs_key op_key = {0};
        int sgn;
        int ret = 0;

        if (extent_op && extent_op->update_key)
                btrfs_disk_key_to_cpu(&op_key, &extent_op->key);

        while ((n = rb_prev(n))) {
                struct btrfs_delayed_ref_node *node;
                node = rb_entry(n, struct btrfs_delayed_ref_node,
                                rb_node);
                if (node->bytenr != head->node.bytenr)
                        break;
                WARN_ON(node->is_head);

                if (node->seq > seq)
                        continue;

                switch (node->action) {
                case BTRFS_ADD_DELAYED_EXTENT:
                case BTRFS_UPDATE_DELAYED_HEAD:
                        WARN_ON(1);
                        continue;
                case BTRFS_ADD_DELAYED_REF:
                        sgn = 1;
                        break;
                case BTRFS_DROP_DELAYED_REF:
                        sgn = -1;
                        break;
                default:
                        BUG_ON(1);
                }
                switch (node->type) {
                case BTRFS_TREE_BLOCK_REF_KEY: {
                        struct btrfs_delayed_tree_ref *ref;

                        ref = btrfs_delayed_node_to_tree_ref(node);
                        ret = __add_prelim_ref(prefs, ref->root, &op_key,
                                               ref->level + 1, 0, node->bytenr,
                                               node->ref_mod * sgn);
                        break;
                }
                case BTRFS_SHARED_BLOCK_REF_KEY: {
                        struct btrfs_delayed_tree_ref *ref;

                        ref = btrfs_delayed_node_to_tree_ref(node);
                        ret = __add_prelim_ref(prefs, ref->root, NULL,
                                               ref->level + 1, ref->parent,
                                               node->bytenr,
                                               node->ref_mod * sgn);
                        break;
                }
                case BTRFS_EXTENT_DATA_REF_KEY: {
                        struct btrfs_delayed_data_ref *ref;
                        ref = btrfs_delayed_node_to_data_ref(node);

                        key.objectid = ref->objectid;
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = ref->offset;
                        ret = __add_prelim_ref(prefs, ref->root, &key, 0, 0,
                                               node->bytenr,
                                               node->ref_mod * sgn);
                        break;
                }
                case BTRFS_SHARED_DATA_REF_KEY: {
                        struct btrfs_delayed_data_ref *ref;

                        ref = btrfs_delayed_node_to_data_ref(node);

                        key.objectid = ref->objectid;
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = ref->offset;
                        ret = __add_prelim_ref(prefs, ref->root, &key, 0,
                                               ref->parent, node->bytenr,
                                               node->ref_mod * sgn);
                        break;
                }
                default:
                        WARN_ON(1);
                }
                BUG_ON(ret);
        }

        return 0;
}

/*
 * add all inline backrefs for bytenr to the list
 */
static int __add_inline_refs(struct btrfs_fs_info *fs_info,
                             struct btrfs_path *path, u64 bytenr,
                             int *info_level, struct list_head *prefs)
{
        int ret = 0;
        int slot;
        struct extent_buffer *leaf;
        struct btrfs_key key;
        unsigned long ptr;
        unsigned long end;
        struct btrfs_extent_item *ei;
        u64 flags;
        u64 item_size;

        /*
         * enumerate all inline refs
         */
        leaf = path->nodes[0];
        slot = path->slots[0];

        item_size = btrfs_item_size_nr(leaf, slot);
        BUG_ON(item_size < sizeof(*ei));

        ei = btrfs_item_ptr(leaf, slot, struct btrfs_extent_item);
        flags = btrfs_extent_flags(leaf, ei);

        ptr = (unsigned long)(ei + 1);
        end = (unsigned long)ei + item_size;

        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                struct btrfs_tree_block_info *info;

                info = (struct btrfs_tree_block_info *)ptr;
                *info_level = btrfs_tree_block_level(leaf, info);
                ptr += sizeof(struct btrfs_tree_block_info);
                BUG_ON(ptr > end);
        } else {
                BUG_ON(!(flags & BTRFS_EXTENT_FLAG_DATA));
        }

        while (ptr < end) {
                struct btrfs_extent_inline_ref *iref;
                u64 offset;
                int type;

                iref = (struct btrfs_extent_inline_ref *)ptr;
                type = btrfs_extent_inline_ref_type(leaf, iref);
                offset = btrfs_extent_inline_ref_offset(leaf, iref);

                switch (type) {
                case BTRFS_SHARED_BLOCK_REF_KEY:
                        ret = __add_prelim_ref(prefs, 0, NULL,
                                                *info_level + 1, offset,
                                                bytenr, 1);
                        break;
                case BTRFS_SHARED_DATA_REF_KEY: {
                        struct btrfs_shared_data_ref *sdref;
                        int count;

                        sdref = (struct btrfs_shared_data_ref *)(iref + 1);
                        count = btrfs_shared_data_ref_count(leaf, sdref);
                        ret = __add_prelim_ref(prefs, 0, NULL, 0, offset,
                                               bytenr, count);
                        break;
                }
                case BTRFS_TREE_BLOCK_REF_KEY:
                        ret = __add_prelim_ref(prefs, offset, NULL,
                                               *info_level + 1, 0,
                                               bytenr, 1);
                        break;
                case BTRFS_EXTENT_DATA_REF_KEY: {
                        struct btrfs_extent_data_ref *dref;
                        int count;
                        u64 root;

                        dref = (struct btrfs_extent_data_ref *)(&iref->offset);
                        count = btrfs_extent_data_ref_count(leaf, dref);
                        key.objectid = btrfs_extent_data_ref_objectid(leaf,
                                                                      dref);
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = btrfs_extent_data_ref_offset(leaf, dref);
                        root = btrfs_extent_data_ref_root(leaf, dref);
                        ret = __add_prelim_ref(prefs, root, &key, 0, 0,
                                               bytenr, count);
                        break;
                }
                default:
                        WARN_ON(1);
                }
                BUG_ON(ret);
                ptr += btrfs_extent_inline_ref_size(type);
        }

        return 0;
}

/*
 * add all non-inline backrefs for bytenr to the list
 */
static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
                            struct btrfs_path *path, u64 bytenr,
                            int info_level, struct list_head *prefs)
{
        struct btrfs_root *extent_root = fs_info->extent_root;
        int ret;
        int slot;
        struct extent_buffer *leaf;
        struct btrfs_key key;

        while (1) {
                ret = btrfs_next_item(extent_root, path);
                if (ret < 0)
                        break;
                if (ret) {
                        ret = 0;
                        break;
                }

                slot = path->slots[0];
                leaf = path->nodes[0];
                btrfs_item_key_to_cpu(leaf, &key, slot);

                if (key.objectid != bytenr)
                        break;
                if (key.type < BTRFS_TREE_BLOCK_REF_KEY)
                        continue;
                if (key.type > BTRFS_SHARED_DATA_REF_KEY)
                        break;

                switch (key.type) {
                case BTRFS_SHARED_BLOCK_REF_KEY:
                        ret = __add_prelim_ref(prefs, 0, NULL,
                                                info_level + 1, key.offset,
                                                bytenr, 1);
                        break;
                case BTRFS_SHARED_DATA_REF_KEY: {
                        struct btrfs_shared_data_ref *sdref;
                        int count;

                        sdref = btrfs_item_ptr(leaf, slot,
                                              struct btrfs_shared_data_ref);
                        count = btrfs_shared_data_ref_count(leaf, sdref);
                        ret = __add_prelim_ref(prefs, 0, NULL, 0, key.offset,
                                                bytenr, count);
                        break;
                }
                case BTRFS_TREE_BLOCK_REF_KEY:
                        ret = __add_prelim_ref(prefs, key.offset, NULL,
                                               info_level + 1, 0,
                                               bytenr, 1);
                        break;
                case BTRFS_EXTENT_DATA_REF_KEY: {
                        struct btrfs_extent_data_ref *dref;
                        int count;
                        u64 root;

                        dref = btrfs_item_ptr(leaf, slot,
                                              struct btrfs_extent_data_ref);
                        count = btrfs_extent_data_ref_count(leaf, dref);
                        key.objectid = btrfs_extent_data_ref_objectid(leaf,
                                                                      dref);
                        key.type = BTRFS_EXTENT_DATA_KEY;
                        key.offset = btrfs_extent_data_ref_offset(leaf, dref);
                        root = btrfs_extent_data_ref_root(leaf, dref);
                        ret = __add_prelim_ref(prefs, root, &key, 0, 0,
                                               bytenr, count);
                        break;
                }
                default:
                        WARN_ON(1);
                }
                BUG_ON(ret);
        }

        return ret;
}

/*
 * this adds all existing backrefs (inline backrefs, backrefs and delayed
 * refs) for the given bytenr to the refs list, merges duplicates and resolves
 * indirect refs to their parent bytenr.
 * When roots are found, they're added to the roots list
 *
 * FIXME some caching might speed things up
 */
static int find_parent_nodes(struct btrfs_trans_handle *trans,
                             struct btrfs_fs_info *fs_info, u64 bytenr,
                             u64 time_seq, struct ulist *refs,
                             struct ulist *roots, const u64 *extent_item_pos)
{
        struct btrfs_key key;
        struct btrfs_path *path;
        struct btrfs_delayed_ref_root *delayed_refs = NULL;
        struct btrfs_delayed_ref_head *head;
        int info_level = 0;
        int ret;
        int search_commit_root = (trans == BTRFS_BACKREF_SEARCH_COMMIT_ROOT);
        struct list_head prefs_delayed;
        struct list_head prefs;
        struct __prelim_ref *ref;

        INIT_LIST_HEAD(&prefs);
        INIT_LIST_HEAD(&prefs_delayed);

        key.objectid = bytenr;
        key.type = BTRFS_EXTENT_ITEM_KEY;
        key.offset = (u64)-1;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;
        path->search_commit_root = !!search_commit_root;

        /*
         * grab both a lock on the path and a lock on the delayed ref head.
         * We need both to get a consistent picture of how the refs look
         * at a specified point in time
         */
again:
        head = NULL;

        ret = btrfs_search_slot(trans, fs_info->extent_root, &key, path, 0, 0);
        if (ret < 0)
                goto out;
        BUG_ON(ret == 0);

        if (trans != BTRFS_BACKREF_SEARCH_COMMIT_ROOT) {
                /*
                 * look if there are updates for this ref queued and lock the
                 * head
                 */
                delayed_refs = &trans->transaction->delayed_refs;
                spin_lock(&delayed_refs->lock);
                head = btrfs_find_delayed_ref_head(trans, bytenr);
                if (head) {
                        if (!mutex_trylock(&head->mutex)) {
                                atomic_inc(&head->node.refs);
                                spin_unlock(&delayed_refs->lock);

                                btrfs_release_path(path);

                                /*
                                 * Mutex was contended, block until it's
                                 * released and try again
                                 */
                                mutex_lock(&head->mutex);
                                mutex_unlock(&head->mutex);
                                btrfs_put_delayed_ref(&head->node);
                                goto again;
                        }
                        ret = __add_delayed_refs(head, time_seq,
                                                 &prefs_delayed);
                        mutex_unlock(&head->mutex);
                        if (ret) {
                                spin_unlock(&delayed_refs->lock);
                                goto out;
                        }
                }
                spin_unlock(&delayed_refs->lock);
        }

        if (path->slots[0]) {
                struct extent_buffer *leaf;
                int slot;

                path->slots[0]--;
                leaf = path->nodes[0];
                slot = path->slots[0];
                btrfs_item_key_to_cpu(leaf, &key, slot);
                if (key.objectid == bytenr &&
                    key.type == BTRFS_EXTENT_ITEM_KEY) {
                        ret = __add_inline_refs(fs_info, path, bytenr,
                                                &info_level, &prefs);
                        if (ret)
                                goto out;
                        ret = __add_keyed_refs(fs_info, path, bytenr,
                                               info_level, &prefs);
                        if (ret)
                                goto out;
                }
        }
        btrfs_release_path(path);

        list_splice_init(&prefs_delayed, &prefs);

        ret = __add_missing_keys(fs_info, &prefs);
        if (ret)
                goto out;

        ret = __merge_refs(&prefs, 1);
        if (ret)
                goto out;

        ret = __resolve_indirect_refs(fs_info, search_commit_root, time_seq,
                                      &prefs, extent_item_pos);
        if (ret)
                goto out;

        ret = __merge_refs(&prefs, 2);
        if (ret)
                goto out;

        while (!list_empty(&prefs)) {
                ref = list_first_entry(&prefs, struct __prelim_ref, list);
                list_del(&ref->list);
                if (ref->count < 0)
                        WARN_ON(1);
                if (ref->count && ref->root_id && ref->parent == 0) {
                        /* no parent == root of tree */
                        ret = ulist_add(roots, ref->root_id, 0, GFP_NOFS);
                        BUG_ON(ret < 0);
                }
                if (ref->count && ref->parent) {
                        struct extent_inode_elem *eie = NULL;
                        if (extent_item_pos && !ref->inode_list) {
                                u32 bsz;
                                struct extent_buffer *eb;
                                bsz = btrfs_level_size(fs_info->extent_root,
                                                        info_level);
                                eb = read_tree_block(fs_info->extent_root,
                                                           ref->parent, bsz, 0);
                                BUG_ON(!eb);
                                ret = find_extent_in_eb(eb, bytenr,
                                                        *extent_item_pos, &eie);
                                ref->inode_list = eie;
                                free_extent_buffer(eb);
                        }
                        ret = ulist_add_merge(refs, ref->parent,
                                              (unsigned long)ref->inode_list,
                                              (unsigned long *)&eie, GFP_NOFS);
                        if (!ret && extent_item_pos) {
                                /*
                                 * we've recorded that parent, so we must extend
                                 * its inode list here
                                 */
                                BUG_ON(!eie);
                                while (eie->next)
                                        eie = eie->next;
                                eie->next = ref->inode_list;
                        }
                        BUG_ON(ret < 0);
                }
                kfree(ref);
        }

out:
        btrfs_free_path(path);
        while (!list_empty(&prefs)) {
                ref = list_first_entry(&prefs, struct __prelim_ref, list);
                list_del(&ref->list);
                kfree(ref);
        }
        while (!list_empty(&prefs_delayed)) {
                ref = list_first_entry(&prefs_delayed, struct __prelim_ref,
                                       list);
                list_del(&ref->list);
                kfree(ref);
        }

        return ret;
}

static void free_leaf_list(struct ulist *blocks)
{
        struct ulist_node *node = NULL;
        struct extent_inode_elem *eie;
        struct extent_inode_elem *eie_next;
        struct ulist_iterator uiter;

        ULIST_ITER_INIT(&uiter);
        while ((node = ulist_next(blocks, &uiter))) {
                if (!node->aux)
                        continue;
                eie = (struct extent_inode_elem *)node->aux;
                for (; eie; eie = eie_next) {
                        eie_next = eie->next;
                        kfree(eie);
                }
                node->aux = 0;
        }

        ulist_free(blocks);
}

/*
 * Finds all leafs with a reference to the specified combination of bytenr and
 * offset. key_list_head will point to a list of corresponding keys (caller must
 * free each list element). The leafs will be stored in the leafs ulist, which
 * must be freed with ulist_free.
 *
 * returns 0 on success, <0 on error
 */
static int btrfs_find_all_leafs(struct btrfs_trans_handle *trans,
                                struct btrfs_fs_info *fs_info, u64 bytenr,
                                u64 time_seq, struct ulist **leafs,
                                const u64 *extent_item_pos)
{
        struct ulist *tmp;
        int ret;

        tmp = ulist_alloc(GFP_NOFS);
        if (!tmp)
                return -ENOMEM;
        *leafs = ulist_alloc(GFP_NOFS);
        if (!*leafs) {
                ulist_free(tmp);
                return -ENOMEM;
        }

        ret = find_parent_nodes(trans, fs_info, bytenr,
                                time_seq, *leafs, tmp, extent_item_pos);
        ulist_free(tmp);

        if (ret < 0 && ret != -ENOENT) {
                free_leaf_list(*leafs);
                return ret;
        }

        return 0;
}

/*
 * walk all backrefs for a given extent to find all roots that reference this
 * extent. Walking a backref means finding all extents that reference this
 * extent and in turn walk the backrefs of those, too. Naturally this is a
 * recursive process, but here it is implemented in an iterative fashion: We
 * find all referencing extents for the extent in question and put them on a
 * list. In turn, we find all referencing extents for those, further appending
 * to the list. The way we iterate the list allows adding more elements after
 * the current while iterating. The process stops when we reach the end of the
 * list. Found roots are added to the roots list.
 *
 * returns 0 on success, < 0 on error.
 */
int btrfs_find_all_roots(struct btrfs_trans_handle *trans,
                                struct btrfs_fs_info *fs_info, u64 bytenr,
                                u64 time_seq, struct ulist **roots)
{
        struct ulist *tmp;
        struct ulist_node *node = NULL;
        struct ulist_iterator uiter;
        int ret;

        tmp = ulist_alloc(GFP_NOFS);
        if (!tmp)
                return -ENOMEM;
        *roots = ulist_alloc(GFP_NOFS);
        if (!*roots) {
                ulist_free(tmp);
                return -ENOMEM;
        }

        ULIST_ITER_INIT(&uiter);
        while (1) {
                ret = find_parent_nodes(trans, fs_info, bytenr,
                                        time_seq, tmp, *roots, NULL);
                if (ret < 0 && ret != -ENOENT) {
                        ulist_free(tmp);
                        ulist_free(*roots);
                        return ret;
                }
                node = ulist_next(tmp, &uiter);
                if (!node)
                        break;
                bytenr = node->val;
        }

        ulist_free(tmp);
        return 0;
}


static int __inode_info(u64 inum, u64 ioff, u8 key_type,
                        struct btrfs_root *fs_root, struct btrfs_path *path,
                        struct btrfs_key *found_key)
{
        int ret;
        struct btrfs_key key;
        struct extent_buffer *eb;

        key.type = key_type;
        key.objectid = inum;
        key.offset = ioff;

        ret = btrfs_search_slot(NULL, fs_root, &key, path, 0, 0);
        if (ret < 0)
                return ret;

        eb = path->nodes[0];
        if (ret && path->slots[0] >= btrfs_header_nritems(eb)) {
                ret = btrfs_next_leaf(fs_root, path);
                if (ret)
                        return ret;
                eb = path->nodes[0];
        }

        btrfs_item_key_to_cpu(eb, found_key, path->slots[0]);
        if (found_key->type != key.type || found_key->objectid != key.objectid)
                return 1;

        return 0;
}

/*
 * this makes the path point to (inum INODE_ITEM ioff)
 */
int inode_item_info(u64 inum, u64 ioff, struct btrfs_root *fs_root,
                        struct btrfs_path *path)
{
        struct btrfs_key key;
        return __inode_info(inum, ioff, BTRFS_INODE_ITEM_KEY, fs_root, path,
                                &key);
}

static int inode_ref_info(u64 inum, u64 ioff, struct btrfs_root *fs_root,
                                struct btrfs_path *path,
                                struct btrfs_key *found_key)
{
        return __inode_info(inum, ioff, BTRFS_INODE_REF_KEY, fs_root, path,
                                found_key);
}

/*
 * this iterates to turn a btrfs_inode_ref into a full filesystem path. elements
 * of the path are separated by '/' and the path is guaranteed to be
 * 0-terminated. the path is only given within the current file system.
 * Therefore, it never starts with a '/'. the caller is responsible to provide
 * "size" bytes in "dest". the dest buffer will be filled backwards. finally,
 * the start point of the resulting string is returned. this pointer is within
 * dest, normally.
 * in case the path buffer would overflow, the pointer is decremented further
 * as if output was written to the buffer, though no more output is actually
 * generated. that way, the caller can determine how much space would be
 * required for the path to fit into the buffer. in that case, the returned
 * value will be smaller than dest. callers must check this!
 */
char *btrfs_iref_to_path(struct btrfs_root *fs_root, struct btrfs_path *path,
                         struct btrfs_inode_ref *iref,
                         struct extent_buffer *eb_in, u64 parent,
                         char *dest, u32 size)
{
        u32 len;
        int slot;
        u64 next_inum;
        int ret;
        s64 bytes_left = size - 1;
        struct extent_buffer *eb = eb_in;
        struct btrfs_key found_key;
        int leave_spinning = path->leave_spinning;

        if (bytes_left >= 0)
                dest[bytes_left] = '\0';

        path->leave_spinning = 1;
        while (1) {
                len = btrfs_inode_ref_name_len(eb, iref);
                bytes_left -= len;
                if (bytes_left >= 0)
                        read_extent_buffer(eb, dest + bytes_left,
                                                (unsigned long)(iref + 1), len);
                if (eb != eb_in) {
                        btrfs_tree_read_unlock_blocking(eb);
                        free_extent_buffer(eb);
                }
                ret = inode_ref_info(parent, 0, fs_root, path, &found_key);
                if (ret > 0)
                        ret = -ENOENT;
                if (ret)
                        break;
                next_inum = found_key.offset;

                /* regular exit ahead */
                if (parent == next_inum)
                        break;

                slot = path->slots[0];
                eb = path->nodes[0];
                /* make sure we can use eb after releasing the path */
                if (eb != eb_in) {
                        atomic_inc(&eb->refs);
                        btrfs_tree_read_lock(eb);
                        btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
                }
                btrfs_release_path(path);

                iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);
                parent = next_inum;
                --bytes_left;
                if (bytes_left >= 0)
                        dest[bytes_left] = '/';
        }

        btrfs_release_path(path);
        path->leave_spinning = leave_spinning;

        if (ret)
                return ERR_PTR(ret);

        return dest + bytes_left;
}

/*
 * this makes the path point to (logical EXTENT_ITEM *)
 * returns BTRFS_EXTENT_FLAG_DATA for data, BTRFS_EXTENT_FLAG_TREE_BLOCK for
 * tree blocks and <0 on error.
 */
int extent_from_logical(struct btrfs_fs_info *fs_info, u64 logical,
                        struct btrfs_path *path, struct btrfs_key *found_key)
{
        int ret;
        u64 flags;
        u32 item_size;
        struct extent_buffer *eb;
        struct btrfs_extent_item *ei;
        struct btrfs_key key;

        key.type = BTRFS_EXTENT_ITEM_KEY;
        key.objectid = logical;
        key.offset = (u64)-1;

        ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
        if (ret < 0)
                return ret;
        ret = btrfs_previous_item(fs_info->extent_root, path,
                                        0, BTRFS_EXTENT_ITEM_KEY);
        if (ret < 0)
                return ret;

        btrfs_item_key_to_cpu(path->nodes[0], found_key, path->slots[0]);
        if (found_key->type != BTRFS_EXTENT_ITEM_KEY ||
            found_key->objectid > logical ||
            found_key->objectid + found_key->offset <= logical) {
                pr_debug("logical %llu is not within any extent\n",
                         (unsigned long long)logical);
                return -ENOENT;
        }

        eb = path->nodes[0];
        item_size = btrfs_item_size_nr(eb, path->slots[0]);
        BUG_ON(item_size < sizeof(*ei));

        ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
        flags = btrfs_extent_flags(eb, ei);

        pr_debug("logical %llu is at position %llu within the extent (%llu "
                 "EXTENT_ITEM %llu) flags %#llx size %u\n",
                 (unsigned long long)logical,
                 (unsigned long long)(logical - found_key->objectid),
                 (unsigned long long)found_key->objectid,
                 (unsigned long long)found_key->offset,
                 (unsigned long long)flags, item_size);
        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
                return BTRFS_EXTENT_FLAG_TREE_BLOCK;
        if (flags & BTRFS_EXTENT_FLAG_DATA)
                return BTRFS_EXTENT_FLAG_DATA;

        return -EIO;
}

/*
 * helper function to iterate extent inline refs. ptr must point to a 0 value
 * for the first call and may be modified. it is used to track state.
 * if more refs exist, 0 is returned and the next call to
 * __get_extent_inline_ref must pass the modified ptr parameter to get the
 * next ref. after the last ref was processed, 1 is returned.
 * returns <0 on error
 */
static int __get_extent_inline_ref(unsigned long *ptr, struct extent_buffer *eb,
                                struct btrfs_extent_item *ei, u32 item_size,
                                struct btrfs_extent_inline_ref **out_eiref,
                                int *out_type)
{
        unsigned long end;
        u64 flags;
        struct btrfs_tree_block_info *info;

        if (!*ptr) {
                /* first call */
                flags = btrfs_extent_flags(eb, ei);
                if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                        info = (struct btrfs_tree_block_info *)(ei + 1);
                        *out_eiref =
                                (struct btrfs_extent_inline_ref *)(info + 1);
                } else {
                        *out_eiref = (struct btrfs_extent_inline_ref *)(ei + 1);
                }
                *ptr = (unsigned long)*out_eiref;
                if ((void *)*ptr >= (void *)ei + item_size)
                        return -ENOENT;
        }

        end = (unsigned long)ei + item_size;
        *out_eiref = (struct btrfs_extent_inline_ref *)*ptr;
        *out_type = btrfs_extent_inline_ref_type(eb, *out_eiref);

        *ptr += btrfs_extent_inline_ref_size(*out_type);
        WARN_ON(*ptr > end);
        if (*ptr == end)
                return 1; /* last */

        return 0;
}

/*
 * reads the tree block backref for an extent. tree level and root are returned
 * through out_level and out_root. ptr must point to a 0 value for the first
 * call and may be modified (see __get_extent_inline_ref comment).
 * returns 0 if data was provided, 1 if there was no more data to provide or
 * <0 on error.
 */
int tree_backref_for_extent(unsigned long *ptr, struct extent_buffer *eb,
                                struct btrfs_extent_item *ei, u32 item_size,
                                u64 *out_root, u8 *out_level)
{
        int ret;
        int type;
        struct btrfs_tree_block_info *info;
        struct btrfs_extent_inline_ref *eiref;

        if (*ptr == (unsigned long)-1)
                return 1;

        while (1) {
                ret = __get_extent_inline_ref(ptr, eb, ei, item_size,
                                                &eiref, &type);
                if (ret < 0)
                        return ret;

                if (type == BTRFS_TREE_BLOCK_REF_KEY ||
                    type == BTRFS_SHARED_BLOCK_REF_KEY)
                        break;

                if (ret == 1)
                        return 1;
        }

        /* we can treat both ref types equally here */
        info = (struct btrfs_tree_block_info *)(ei + 1);
        *out_root = btrfs_extent_inline_ref_offset(eb, eiref);
        *out_level = btrfs_tree_block_level(eb, info);

        if (ret == 1)
                *ptr = (unsigned long)-1;

        return 0;
}

static int iterate_leaf_refs(struct extent_inode_elem *inode_list,
                                u64 root, u64 extent_item_objectid,
                                iterate_extent_inodes_t *iterate, void *ctx)
{
        struct extent_inode_elem *eie;
        int ret = 0;

        for (eie = inode_list; eie; eie = eie->next) {
                pr_debug("ref for %llu resolved, key (%llu EXTEND_DATA %llu), "
                         "root %llu\n", extent_item_objectid,
                         eie->inum, eie->offset, root);
                ret = iterate(eie->inum, eie->offset, root, ctx);
                if (ret) {
                        pr_debug("stopping iteration for %llu due to ret=%d\n",
                                 extent_item_objectid, ret);
                        break;
                }
        }

        return ret;
}

/*
 * calls iterate() for every inode that references the extent identified by
 * the given parameters.
 * when the iterator function returns a non-zero value, iteration stops.
 */
int iterate_extent_inodes(struct btrfs_fs_info *fs_info,
                                u64 extent_item_objectid, u64 extent_item_pos,
                                int search_commit_root,
                                iterate_extent_inodes_t *iterate, void *ctx)
{
        int ret;
        struct list_head data_refs = LIST_HEAD_INIT(data_refs);
        struct list_head shared_refs = LIST_HEAD_INIT(shared_refs);
        struct btrfs_trans_handle *trans;
        struct ulist *refs = NULL;
        struct ulist *roots = NULL;
        struct ulist_node *ref_node = NULL;
        struct ulist_node *root_node = NULL;
        struct seq_list tree_mod_seq_elem = {};
        struct ulist_iterator ref_uiter;
        struct ulist_iterator root_uiter;

        pr_debug("resolving all inodes for extent %llu\n",
                        extent_item_objectid);

        if (search_commit_root) {
                trans = BTRFS_BACKREF_SEARCH_COMMIT_ROOT;
        } else {
                trans = btrfs_join_transaction(fs_info->extent_root);
                if (IS_ERR(trans))
                        return PTR_ERR(trans);
                btrfs_get_tree_mod_seq(fs_info, &tree_mod_seq_elem);
        }

        ret = btrfs_find_all_leafs(trans, fs_info, extent_item_objectid,
                                   tree_mod_seq_elem.seq, &refs,
                                   &extent_item_pos);
        if (ret)
                goto out;

        ULIST_ITER_INIT(&ref_uiter);
        while (!ret && (ref_node = ulist_next(refs, &ref_uiter))) {
                ret = btrfs_find_all_roots(trans, fs_info, ref_node->val,
                                           tree_mod_seq_elem.seq, &roots);
                if (ret)
                        break;
                ULIST_ITER_INIT(&root_uiter);
                while (!ret && (root_node = ulist_next(roots, &root_uiter))) {
                        pr_debug("root %llu references leaf %llu, data list "
                                 "%#lx\n", root_node->val, ref_node->val,
                                 ref_node->aux);
                        ret = iterate_leaf_refs(
                                (struct extent_inode_elem *)ref_node->aux,
                                root_node->val, extent_item_objectid,
                                iterate, ctx);
                }
                ulist_free(roots);
                roots = NULL;
        }

        free_leaf_list(refs);
        ulist_free(roots);
out:
        if (!search_commit_root) {
                btrfs_put_tree_mod_seq(fs_info, &tree_mod_seq_elem);
                btrfs_end_transaction(trans, fs_info->extent_root);
        }

        return ret;
}

int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
                                struct btrfs_path *path,
                                iterate_extent_inodes_t *iterate, void *ctx)
{
        int ret;
        u64 extent_item_pos;
        struct btrfs_key found_key;
        int search_commit_root = path->search_commit_root;

        ret = extent_from_logical(fs_info, logical, path,
                                        &found_key);
        btrfs_release_path(path);
        if (ret < 0)
                return ret;
        if (ret & BTRFS_EXTENT_FLAG_TREE_BLOCK)
                return -EINVAL;

        extent_item_pos = logical - found_key.objectid;
        ret = iterate_extent_inodes(fs_info, found_key.objectid,
                                        extent_item_pos, search_commit_root,
                                        iterate, ctx);

        return ret;
}

static int iterate_irefs(u64 inum, struct btrfs_root *fs_root,
                                struct btrfs_path *path,
                                iterate_irefs_t *iterate, void *ctx)
{
        int ret = 0;
        int slot;
        u32 cur;
        u32 len;
        u32 name_len;
        u64 parent = 0;
        int found = 0;
        struct extent_buffer *eb;
        struct btrfs_item *item;
        struct btrfs_inode_ref *iref;
        struct btrfs_key found_key;

        while (!ret) {
                path->leave_spinning = 1;
                ret = inode_ref_info(inum, parent ? parent+1 : 0, fs_root, path,
                                        &found_key);
                if (ret < 0)
                        break;
                if (ret) {
                        ret = found ? 0 : -ENOENT;
                        break;
                }
                ++found;

                parent = found_key.offset;
                slot = path->slots[0];
                eb = path->nodes[0];
                /* make sure we can use eb after releasing the path */
                atomic_inc(&eb->refs);
                btrfs_tree_read_lock(eb);
                btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
                btrfs_release_path(path);

                item = btrfs_item_nr(eb, slot);
                iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);

                for (cur = 0; cur < btrfs_item_size(eb, item); cur += len) {
                        name_len = btrfs_inode_ref_name_len(eb, iref);
                        /* path must be released before calling iterate()! */
                        pr_debug("following ref at offset %u for inode %llu in "
                                 "tree %llu\n", cur,
                                 (unsigned long long)found_key.objectid,
                                 (unsigned long long)fs_root->objectid);
                        ret = iterate(parent, iref, eb, ctx);
                        if (ret)
                                break;
                        len = sizeof(*iref) + name_len;
                        iref = (struct btrfs_inode_ref *)((char *)iref + len);
                }
                btrfs_tree_read_unlock_blocking(eb);
                free_extent_buffer(eb);
        }

        btrfs_release_path(path);

        return ret;
}

/*
 * returns 0 if the path could be dumped (probably truncated)
 * returns <0 in case of an error
 */
static int inode_to_path(u64 inum, struct btrfs_inode_ref *iref,
                                struct extent_buffer *eb, void *ctx)
{
        struct inode_fs_paths *ipath = ctx;
        char *fspath;
        char *fspath_min;
        int i = ipath->fspath->elem_cnt;
        const int s_ptr = sizeof(char *);
        u32 bytes_left;

        bytes_left = ipath->fspath->bytes_left > s_ptr ?
                                        ipath->fspath->bytes_left - s_ptr : 0;

        fspath_min = (char *)ipath->fspath->val + (i + 1) * s_ptr;
        fspath = btrfs_iref_to_path(ipath->fs_root, ipath->btrfs_path, iref, eb,
                                inum, fspath_min, bytes_left);
        if (IS_ERR(fspath))
                return PTR_ERR(fspath);

        if (fspath > fspath_min) {
                pr_debug("path resolved: %s\n", fspath);
                ipath->fspath->val[i] = (u64)(unsigned long)fspath;
                ++ipath->fspath->elem_cnt;
                ipath->fspath->bytes_left = fspath - fspath_min;
        } else {
                pr_debug("missed path, not enough space. missing bytes: %lu, "
                         "constructed so far: %s\n",
                         (unsigned long)(fspath_min - fspath), fspath_min);
                ++ipath->fspath->elem_missed;
                ipath->fspath->bytes_missing += fspath_min - fspath;
                ipath->fspath->bytes_left = 0;
        }

        return 0;
}

/*
 * this dumps all file system paths to the inode into the ipath struct, provided
 * is has been created large enough. each path is zero-terminated and accessed
 * from ipath->fspath->val[i].
 * when it returns, there are ipath->fspath->elem_cnt number of paths available
 * in ipath->fspath->val[]. when the allocated space wasn't sufficient, the
 * number of missed paths in recored in ipath->fspath->elem_missed, otherwise,
 * it's zero. ipath->fspath->bytes_missing holds the number of bytes that would
 * have been needed to return all paths.
 */
int paths_from_inode(u64 inum, struct inode_fs_paths *ipath)
{
        return iterate_irefs(inum, ipath->fs_root, ipath->btrfs_path,
                                inode_to_path, ipath);
}

struct btrfs_data_container *init_data_container(u32 total_bytes)
{
        struct btrfs_data_container *data;
        size_t alloc_bytes;

        alloc_bytes = max_t(size_t, total_bytes, sizeof(*data));
        data = kmalloc(alloc_bytes, GFP_NOFS);
        if (!data)
                return ERR_PTR(-ENOMEM);

        if (total_bytes >= sizeof(*data)) {
                data->bytes_left = total_bytes - sizeof(*data);
                data->bytes_missing = 0;
        } else {
                data->bytes_missing = sizeof(*data) - total_bytes;
                data->bytes_left = 0;
        }

        data->elem_cnt = 0;
        data->elem_missed = 0;

        return data;
}

/*
 * allocates space to return multiple file system paths for an inode.
 * total_bytes to allocate are passed, note that space usable for actual path
 * information will be total_bytes - sizeof(struct inode_fs_paths).
 * the returned pointer must be freed with free_ipath() in the end.
 */
struct inode_fs_paths *init_ipath(s32 total_bytes, struct btrfs_root *fs_root,
                                        struct btrfs_path *path)
{
        struct inode_fs_paths *ifp;
        struct btrfs_data_container *fspath;

        fspath = init_data_container(total_bytes);
        if (IS_ERR(fspath))
                return (void *)fspath;

        ifp = kmalloc(sizeof(*ifp), GFP_NOFS);
        if (!ifp) {
                kfree(fspath);
                return ERR_PTR(-ENOMEM);
        }

        ifp->btrfs_path = path;
        ifp->fspath = fspath;
        ifp->fs_root = fs_root;

        return ifp;
}

void free_ipath(struct inode_fs_paths *ipath)
{
        if (!ipath)
                return;
        kfree(ipath->fspath);
        kfree(ipath);
}