BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE
for (int i = 0; i < BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE; i++) {
slot = (slot + 1) % BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE;
} prev_extents_cache[BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE];