osd_fallocate_preallocate() advances the caller's byte offset *start by the block count from ldiskfs_map_blocks() ("*start += rc;", osd_io.c). *start is ofd_object_fallocate()'s resume point across its -EAGAIN restart loop, so a restart resumes ~4096x too early and re-maps what it already mapped.

Found by graph-hunter-v1