[PATCH v6 01/24] erofs: add on-disk layout

[Christoph Hellwig]

On Sun, Sep 01, 2019 at 03:54:11PM +0800, Gao Xiang wrote:
> It could be better has a name though, because 1) erofs.mkfs uses this
> definition explicitly, and we keep this on-disk definition erofs_fs.h
> file up with erofs-utils.
> 
> 2) For kernel use, first we have,
>    datamode < EROFS_INODE_LAYOUT_MAX; and
>    !erofs_inode_is_data_compressed, so there are only two mode here,
>         1) EROFS_INODE_FLAT_INLINE,
>         2) EROFS_INODE_FLAT_PLAIN
>    if its datamode isn't EROFS_INODE_FLAT_INLINE (tail-end block packing),
>    it should be EROFS_INODE_FLAT_PLAIN.
> 
>    The detailed logic in erofs_read_inode and
>    erofs_map_blocks_flatmode....

Ok.  At least the explicit numbering makes this a little more obvious
now.  What seems fairly odd is that there are only various places that
check for some inode layouts/formats but nothing that does a switch
over all of them.

> > why are we adding a legacy field to a brand new file system?
> 
> The difference is just EROFS_INODE_FLAT_COMPRESSION_LEGACY doesn't
> have z_erofs_map_header, so it only supports default (4k clustersize)
> fixed-sized output compression rather than per-file setting, nothing
> special at all...

It still seems odd to add a legacy field to a brand new file system.

> > structures, as that keeps it clear in everyones mind what needs to
> > stay persistent and what can be chenged easily.
> 
> All fields in this file are on-disk representation by design
> (no logic for in-memory presentation).

Ok, make sense.    Maybe add a note to the top of the file comment
that this is the on-disk format.

One little oddity is that erofs_inode_is_data_compressed is here, while
is_inode_flat_inline is in internal.h.  There are arguments for either
place, but I'd suggest to keep the related macros together.