Btrfs: add helper for tree enumeration

Often no exact match is wanted but just the next lower or
higher item. There's a lot of duplicated code throughout
btrfs to deal with the corner cases. This patch adds a
helper function that can facilitate searching.

Signed-off-by: Arne Jansen <sensille@gmx.net>

2 files changed, 75 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index bef68ab32204..fb21431fe4e0 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2789,6 +2789,78 @@ done:
 }
 
 /*
+ * helper to use instead of search slot if no exact match is needed but
+ * instead the next or previous item should be returned.
+ * When find_higher is true, the next higher item is returned, the next lower
+ * otherwise.
+ * When return_any and find_higher are both true, and no higher item is found,
+ * return the next lower instead.
+ * When return_any is true and find_higher is false, and no lower item is found,
+ * return the next higher instead.
+ * It returns 0 if any item is found, 1 if none is found (tree empty), and
+ * < 0 on error
+ */
+int btrfs_search_slot_for_read(struct btrfs_root *root,
+                               struct btrfs_key *key, struct btrfs_path *p,
+                               int find_higher, int return_any)
+{
+        int ret;
+        struct extent_buffer *leaf;
+
+again:
+        ret = btrfs_search_slot(NULL, root, key, p, 0, 0);
+        if (ret <= 0)
+                return ret;
+        /*
+         * a return value of 1 means the path is at the position where the
+         * item should be inserted. Normally this is the next bigger item,
+         * but in case the previous item is the last in a leaf, path points
+         * to the first free slot in the previous leaf, i.e. at an invalid
+         * item.
+         */
+        leaf = p->nodes[0];
+
+        if (find_higher) {
+                if (p->slots[0] >= btrfs_header_nritems(leaf)) {
+                        ret = btrfs_next_leaf(root, p);
+                        if (ret <= 0)
+                                return ret;
+                        if (!return_any)
+                                return 1;
+                        /*
+                         * no higher item found, return the next
+                         * lower instead
+                         */
+                        return_any = 0;
+                        find_higher = 0;
+                        btrfs_release_path(p);
+                        goto again;
+                }
+        } else {
+                if (p->slots[0] >= btrfs_header_nritems(leaf)) {
+                        /* we're sitting on an invalid slot */
+                        if (p->slots[0] == 0) {
+                                ret = btrfs_prev_leaf(root, p);
+                                if (ret <= 0)
+                                        return ret;
+                                if (!return_any)
+                                        return 1;
+                                /*
+                                 * no lower item found, return the next
+                                 * higher instead
+                                 */
+                                return_any = 0;
+                                find_higher = 1;
+                                btrfs_release_path(p);
+                                goto again;
+                        }
+                        --p->slots[0];
+                }
+        }
+        return 0;
+}
+
+/*
  * adjust the pointers going up the tree, starting at level
  * making sure the right key of each node is points to 'key'.
  * This is used after shifting pointers to the left, so it stops
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 33088b0dbf3f..27cf995564ed 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2856,6 +2856,9 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
                       ins_len, int cow);
 int btrfs_search_old_slot(struct btrfs_root *root, struct btrfs_key *key,
                           struct btrfs_path *p, u64 time_seq);
+int btrfs_search_slot_for_read(struct btrfs_root *root,
+                               struct btrfs_key *key, struct btrfs_path *p,
+                               int find_higher, int return_any);
 int btrfs_realloc_node(struct btrfs_trans_handle *trans,
                        struct btrfs_root *root, struct extent_buffer *parent,
                        int start_slot, int cache_only, u64 *last_ret,