2 files changed, 440 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index c82a9e4a953..4c10fd19d48 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -5005,6 +5005,431 @@ out:
         return ret;
 }
 
+static void tree_move_down(struct btrfs_root *root,
+                           struct btrfs_path *path,
+                           int *level, int root_level)
+{
+        path->nodes[*level - 1] = read_node_slot(root, path->nodes[*level],
+                                        path->slots[*level]);
+        path->slots[*level - 1] = 0;
+        (*level)--;
+}
+
+static int tree_move_next_or_upnext(struct btrfs_root *root,
+                                    struct btrfs_path *path,
+                                    int *level, int root_level)
+{
+        int ret = 0;
+        int nritems;
+        nritems = btrfs_header_nritems(path->nodes[*level]);
+
+        path->slots[*level]++;
+
+        while (path->slots[*level] == nritems) {
+                if (*level == root_level)
+                        return -1;
+
+                /* move upnext */
+                path->slots[*level] = 0;
+                free_extent_buffer(path->nodes[*level]);
+                path->nodes[*level] = NULL;
+                (*level)++;
+                path->slots[*level]++;
+
+                nritems = btrfs_header_nritems(path->nodes[*level]);
+                ret = 1;
+        }
+        return ret;
+}
+
+/*
+ * Returns 1 if it had to move up and next. 0 is returned if it moved only next
+ * or down.
+ */
+static int tree_advance(struct btrfs_root *root,
+                        struct btrfs_path *path,
+                        int *level, int root_level,
+                        int allow_down,
+                        struct btrfs_key *key)
+{
+        int ret;
+
+        if (*level == 0 || !allow_down) {
+                ret = tree_move_next_or_upnext(root, path, level, root_level);
+        } else {
+                tree_move_down(root, path, level, root_level);
+                ret = 0;
+        }
+        if (ret >= 0) {
+                if (*level == 0)
+                        btrfs_item_key_to_cpu(path->nodes[*level], key,
+                                        path->slots[*level]);
+                else
+                        btrfs_node_key_to_cpu(path->nodes[*level], key,
+                                        path->slots[*level]);
+        }
+        return ret;
+}
+
+static int tree_compare_item(struct btrfs_root *left_root,
+                             struct btrfs_path *left_path,
+                             struct btrfs_path *right_path,
+                             char *tmp_buf)
+{
+        int cmp;
+        int len1, len2;
+        unsigned long off1, off2;
+
+        len1 = btrfs_item_size_nr(left_path->nodes[0], left_path->slots[0]);
+        len2 = btrfs_item_size_nr(right_path->nodes[0], right_path->slots[0]);
+        if (len1 != len2)
+                return 1;
+
+        off1 = btrfs_item_ptr_offset(left_path->nodes[0], left_path->slots[0]);
+        off2 = btrfs_item_ptr_offset(right_path->nodes[0],
+                                right_path->slots[0]);
+
+        read_extent_buffer(left_path->nodes[0], tmp_buf, off1, len1);
+
+        cmp = memcmp_extent_buffer(right_path->nodes[0], tmp_buf, off2, len1);
+        if (cmp)
+                return 1;
+        return 0;
+}
+
+#define ADVANCE 1
+#define ADVANCE_ONLY_NEXT -1
+
+/*
+ * This function compares two trees and calls the provided callback for
+ * every changed/new/deleted item it finds.
+ * If shared tree blocks are encountered, whole subtrees are skipped, making
+ * the compare pretty fast on snapshotted subvolumes.
+ *
+ * This currently works on commit roots only. As commit roots are read only,
+ * we don't do any locking. The commit roots are protected with transactions.
+ * Transactions are ended and rejoined when a commit is tried in between.
+ *
+ * This function checks for modifications done to the trees while comparing.
+ * If it detects a change, it aborts immediately.
+ */
+int btrfs_compare_trees(struct btrfs_root *left_root,
+                        struct btrfs_root *right_root,
+                        btrfs_changed_cb_t changed_cb, void *ctx)
+{
+        int ret;
+        int cmp;
+        struct btrfs_trans_handle *trans = NULL;
+        struct btrfs_path *left_path = NULL;
+        struct btrfs_path *right_path = NULL;
+        struct btrfs_key left_key;
+        struct btrfs_key right_key;
+        char *tmp_buf = NULL;
+        int left_root_level;
+        int right_root_level;
+        int left_level;
+        int right_level;
+        int left_end_reached;
+        int right_end_reached;
+        int advance_left;
+        int advance_right;
+        u64 left_blockptr;
+        u64 right_blockptr;
+        u64 left_start_ctransid;
+        u64 right_start_ctransid;
+        u64 ctransid;
+
+        left_path = btrfs_alloc_path();
+        if (!left_path) {
+                ret = -ENOMEM;
+                goto out;
+        }
+        right_path = btrfs_alloc_path();
+        if (!right_path) {
+                ret = -ENOMEM;
+                goto out;
+        }
+
+        tmp_buf = kmalloc(left_root->leafsize, GFP_NOFS);
+        if (!tmp_buf) {
+                ret = -ENOMEM;
+                goto out;
+        }
+
+        left_path->search_commit_root = 1;
+        left_path->skip_locking = 1;
+        right_path->search_commit_root = 1;
+        right_path->skip_locking = 1;
+
+        spin_lock(&left_root->root_times_lock);
+        left_start_ctransid = btrfs_root_ctransid(&left_root->root_item);
+        spin_unlock(&left_root->root_times_lock);
+
+        spin_lock(&right_root->root_times_lock);
+        right_start_ctransid = btrfs_root_ctransid(&right_root->root_item);
+        spin_unlock(&right_root->root_times_lock);
+
+        trans = btrfs_join_transaction(left_root);
+        if (IS_ERR(trans)) {
+                ret = PTR_ERR(trans);
+                trans = NULL;
+                goto out;
+        }
+
+        /*
+         * Strategy: Go to the first items of both trees. Then do
+         *
+         * If both trees are at level 0
+         *   Compare keys of current items
+         *     If left < right treat left item as new, advance left tree
+         *       and repeat
+         *     If left > right treat right item as deleted, advance right tree
+         *       and repeat
+         *     If left == right do deep compare of items, treat as changed if
+         *       needed, advance both trees and repeat
+         * If both trees are at the same level but not at level 0
+         *   Compare keys of current nodes/leafs
+         *     If left < right advance left tree and repeat
+         *     If left > right advance right tree and repeat
+         *     If left == right compare blockptrs of the next nodes/leafs
+         *       If they match advance both trees but stay at the same level
+         *         and repeat
+         *       If they don't match advance both trees while allowing to go
+         *         deeper and repeat
+         * If tree levels are different
+         *   Advance the tree that needs it and repeat
+         *
+         * Advancing a tree means:
+         *   If we are at level 0, try to go to the next slot. If that's not
+         *   possible, go one level up and repeat. Stop when we found a level
+         *   where we could go to the next slot. We may at this point be on a
+         *   node or a leaf.
+         *
+         *   If we are not at level 0 and not on shared tree blocks, go one
+         *   level deeper.
+         *
+         *   If we are not at level 0 and on shared tree blocks, go one slot to
+         *   the right if possible or go up and right.
+         */
+
+        left_level = btrfs_header_level(left_root->commit_root);
+        left_root_level = left_level;
+        left_path->nodes[left_level] = left_root->commit_root;
+        extent_buffer_get(left_path->nodes[left_level]);
+
+        right_level = btrfs_header_level(right_root->commit_root);
+        right_root_level = right_level;
+        right_path->nodes[right_level] = right_root->commit_root;
+        extent_buffer_get(right_path->nodes[right_level]);
+
+        if (left_level == 0)
+                btrfs_item_key_to_cpu(left_path->nodes[left_level],
+                                &left_key, left_path->slots[left_level]);
+        else
+                btrfs_node_key_to_cpu(left_path->nodes[left_level],
+                                &left_key, left_path->slots[left_level]);
+        if (right_level == 0)
+                btrfs_item_key_to_cpu(right_path->nodes[right_level],
+                                &right_key, right_path->slots[right_level]);
+        else
+                btrfs_node_key_to_cpu(right_path->nodes[right_level],
+                                &right_key, right_path->slots[right_level]);
+
+        left_end_reached = right_end_reached = 0;
+        advance_left = advance_right = 0;
+
+        while (1) {
+                /*
+                 * We need to make sure the transaction does not get committed
+                 * while we do anything on commit roots. This means, we need to
+                 * join and leave transactions for every item that we process.
+                 */
+                if (trans && btrfs_should_end_transaction(trans, left_root)) {
+                        btrfs_release_path(left_path);
+                        btrfs_release_path(right_path);
+
+                        ret = btrfs_end_transaction(trans, left_root);
+                        trans = NULL;
+                        if (ret < 0)
+                                goto out;
+                }
+                /* now rejoin the transaction */
+                if (!trans) {
+                        trans = btrfs_join_transaction(left_root);
+                        if (IS_ERR(trans)) {
+                                ret = PTR_ERR(trans);
+                                trans = NULL;
+                                goto out;
+                        }
+
+                        spin_lock(&left_root->root_times_lock);
+                        ctransid = btrfs_root_ctransid(&left_root->root_item);
+                        spin_unlock(&left_root->root_times_lock);
+                        if (ctransid != left_start_ctransid)
+                                left_start_ctransid = 0;
+
+                        spin_lock(&right_root->root_times_lock);
+                        ctransid = btrfs_root_ctransid(&right_root->root_item);
+                        spin_unlock(&right_root->root_times_lock);
+                        if (ctransid != right_start_ctransid)
+                                right_start_ctransid = 0;
+
+                        if (!left_start_ctransid || !right_start_ctransid) {
+                                WARN(1, KERN_WARNING
+                                        "btrfs: btrfs_compare_tree detected "
+                                        "a change in one of the trees while "
+                                        "iterating. This is probably a "
+                                        "bug.\n");
+                                ret = -EIO;
+                                goto out;
+                        }
+
+                        /*
+                         * the commit root may have changed, so start again
+                         * where we stopped
+                         */
+                        left_path->lowest_level = left_level;
+                        right_path->lowest_level = right_level;
+                        ret = btrfs_search_slot(NULL, left_root,
+                                        &left_key, left_path, 0, 0);
+                        if (ret < 0)
+                                goto out;
+                        ret = btrfs_search_slot(NULL, right_root,
+                                        &right_key, right_path, 0, 0);
+                        if (ret < 0)
+                                goto out;
+                }
+
+                if (advance_left && !left_end_reached) {
+                        ret = tree_advance(left_root, left_path, &left_level,
+                                        left_root_level,
+                                        advance_left != ADVANCE_ONLY_NEXT,
+                                        &left_key);
+                        if (ret < 0)
+                                left_end_reached = ADVANCE;
+                        advance_left = 0;
+                }
+                if (advance_right && !right_end_reached) {
+                        ret = tree_advance(right_root, right_path, &right_level,
+                                        right_root_level,
+                                        advance_right != ADVANCE_ONLY_NEXT,
+                                        &right_key);
+                        if (ret < 0)
+                                right_end_reached = ADVANCE;
+                        advance_right = 0;
+                }
+
+                if (left_end_reached && right_end_reached) {
+                        ret = 0;
+                        goto out;
+                } else if (left_end_reached) {
+                        if (right_level == 0) {
+                                ret = changed_cb(left_root, right_root,
+                                                left_path, right_path,
+                                                &right_key,
+                                                BTRFS_COMPARE_TREE_DELETED,
+                                                ctx);
+                                if (ret < 0)
+                                        goto out;
+                        }
+                        advance_right = ADVANCE;
+                        continue;
+                } else if (right_end_reached) {
+                        if (left_level == 0) {
+                                ret = changed_cb(left_root, right_root,
+                                                left_path, right_path,
+                                                &left_key,
+                                                BTRFS_COMPARE_TREE_NEW,
+                                                ctx);
+                                if (ret < 0)
+                                        goto out;
+                        }
+                        advance_left = ADVANCE;
+                        continue;
+                }
+
+                if (left_level == 0 && right_level == 0) {
+                        cmp = btrfs_comp_cpu_keys(&left_key, &right_key);
+                        if (cmp < 0) {
+                                ret = changed_cb(left_root, right_root,
+                                                left_path, right_path,
+                                                &left_key,
+                                                BTRFS_COMPARE_TREE_NEW,
+                                                ctx);
+                                if (ret < 0)
+                                        goto out;
+                                advance_left = ADVANCE;
+                        } else if (cmp > 0) {
+                                ret = changed_cb(left_root, right_root,
+                                                left_path, right_path,
+                                                &right_key,
+                                                BTRFS_COMPARE_TREE_DELETED,
+                                                ctx);
+                                if (ret < 0)
+                                        goto out;
+                                advance_right = ADVANCE;
+                        } else {
+                                ret = tree_compare_item(left_root, left_path,
+                                                right_path, tmp_buf);
+                                if (ret) {
+                                        ret = changed_cb(left_root, right_root,
+                                                left_path, right_path,
+                                                &left_key,
+                                                BTRFS_COMPARE_TREE_CHANGED,
+                                                ctx);
+                                        if (ret < 0)
+                                                goto out;
+                                }
+                                advance_left = ADVANCE;
+                                advance_right = ADVANCE;
+                        }
+                } else if (left_level == right_level) {
+                        cmp = btrfs_comp_cpu_keys(&left_key, &right_key);
+                        if (cmp < 0) {
+                                advance_left = ADVANCE;
+                        } else if (cmp > 0) {
+                                advance_right = ADVANCE;
+                        } else {
+                                left_blockptr = btrfs_node_blockptr(
+                                                left_path->nodes[left_level],
+                                                left_path->slots[left_level]);
+                                right_blockptr = btrfs_node_blockptr(
+                                                right_path->nodes[right_level],
+                                                right_path->slots[right_level]);
+                                if (left_blockptr == right_blockptr) {
+                                        /*
+                                         * As we're on a shared block, don't
+                                         * allow to go deeper.
+                                         */
+                                        advance_left = ADVANCE_ONLY_NEXT;
+                                        advance_right = ADVANCE_ONLY_NEXT;
+                                } else {
+                                        advance_left = ADVANCE;
+                                        advance_right = ADVANCE;
+                                }
+                        }
+                } else if (left_level < right_level) {
+                        advance_right = ADVANCE;
+                } else {
+                        advance_left = ADVANCE;
+                }
+        }
+
+out:
+        btrfs_free_path(left_path);
+        btrfs_free_path(right_path);
+        kfree(tmp_buf);
+
+        if (trans) {
+                if (!ret)
+                        ret = btrfs_end_transaction(trans, left_root);
+                else
+                        btrfs_end_transaction(trans, left_root);
+        }
+
+        return ret;
+}
+
 /*
  * this is similar to btrfs_next_leaf, but does not try to preserve
  * and fixup the path.  It looks for and returns the next key in the
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index d5f6d745867..2fbbe738cae 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2722,6 +2722,21 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
                          struct btrfs_key *max_key,
                          struct btrfs_path *path, int cache_only,
                          u64 min_trans);
+enum btrfs_compare_tree_result {
+        BTRFS_COMPARE_TREE_NEW,
+        BTRFS_COMPARE_TREE_DELETED,
+        BTRFS_COMPARE_TREE_CHANGED,
+};
+typedef int (*btrfs_changed_cb_t)(struct btrfs_root *left_root,
+                                  struct btrfs_root *right_root,
+                                  struct btrfs_path *left_path,
+                                  struct btrfs_path *right_path,
+                                  struct btrfs_key *key,
+                                  enum btrfs_compare_tree_result result,
+                                  void *ctx);
+int btrfs_compare_trees(struct btrfs_root *left_root,
+                        struct btrfs_root *right_root,
+                        btrfs_changed_cb_t cb, void *ctx);
 int btrfs_cow_block(struct btrfs_trans_handle *trans,
                     struct btrfs_root *root, struct extent_buffer *buf,
                     struct extent_buffer *parent, int parent_slot,