Re: Rebuild mode for tail-pack layouts

[Gao Xiang <xiang@kernel.org>]

On Sat, May 09, 2026 at 04:37:43PM +0800, Bingwu Zhang wrote:
> 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?

Yes, sorry I didn't express explicitly.

I mean the main data except the trailing inline data part
can be remapped into another external blob.

But inline data should be kept with the metadata; otherwise it
won't be called _inline data_ anymore.

Or do you have a case we have to redirect the inline data?

Thanks,
Gao Xiang

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