Rebuild mode for tail-pack layouts

Hi!

In erofs_rebuild_write_blob_index (rebuild.c), only EROFS_INODE_CHUNK_BASED 
and FLAT_PLAIN are implemented, so when generating a metadata index with 
rebuild mode, the sources cannot use tail-pack nor inline data layout.
However, disabling tail-packing can lead to great disk-space waste in many 
cases, especially when the file-system consists of a lot of small files.

Thus I attempted to implement FLAT_INLINE for it, only to realize that the 
current chunk entry formats can only represent physical addresses that are 
block-aligned while tail-pack extent is not.

I wonder what do you think about adding a new chunk entry format? And how 
should it be named?

I would suggest the following structure:
struct erofs_inode_chunk_index_tp {
	__le16 startblk_hi;	/* starting block number MSB */
	__le16 device_id;	/* back-end storage id (with bits masked) 
*/
	__le32 startblk_lo;	/* starting block number of this chunk */
	/* new fields below */
	__le16 startblk_off;	/* starting block offset */
	__le16 reserved;
} __packed;
The 16b offset should be enough unless we are to support block size > 64K.
The reserved field is added for alignment.

Best wishes.

-- 
xtex (Bingwu Zhang, a.k.a Zerogawa Chiaki) @ Thu, 30 Apr 2026 11:45:17 +0000