Re: Rebuild mode for tail-pack layouts

[Bingwu Zhang <xtex@envs.net>]

Hi,

On Monday, May 4, 2026 11:13:23 PM China Standard Time Gao Xiang wrote:
> Hi xtex,
> 
> On 2026/4/30 20:09, xtex wrote:
> > 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.
> 
> Sorry about the late response.
> 
> Thanks for the question.
> 
> FLAT_INLINE can be used for index rebuilding, which can work with
> uniaddr (or mapped_blkaddr) since the blkaddr will be mapped
> into the relative address based on the blob starting with
> mapped_blkaddr:
> 
> https://erofs.docs.kernel.org/en/latest/ondisk/chunked_format.html#device-ta
> ble
> 
> But I agree the expression in the page above is a bit
> ambigious through.
> 
> Thanks,
> Gao Xiang
> 
> > Best wishes.

Sorry for the late response and thanks for your answer.

I am sorry about that I didn't get it.
In __erofs_map_blocks:
> map->m_pa = erofs_pos(sbi, startblk);
and
> #define erofs_pos(sbi, nr)      ((erofs_off_t)(nr) << (sbi)->blkszbits)
It seems like that the mapped PA is always block-aligned? However, the last 
chunk of inline data is not?

Best wishes.

-- 
xtex (a.k.a. Bingwu Zhang) @ Sat, 09 May 2026 08:24:53 +0000