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SubjectRe: [PATCH v6 01/24] erofs: add on-disk layout
Hi Christoph,

On Mon, Sep 02, 2019 at 05:45:21AM -0700, Christoph Hellwig wrote:
> 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.

(Maybe not explicitly for this part....)

erofs_map_blocks_flatmode()
...
97 nblocks = DIV_ROUND_UP(inode->i_size, PAGE_SIZE);
98 lastblk = nblocks - is_inode_flat_inline(inode);
^ here
...

Believe me EROFS_INODE_FLAT_PLAIN is used widely for EROFS images....
(if EROFS_INODE_FLAT_INLINE tail-end packing is not suitable and
no compression....)

>
> > > 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.

Since 4.19 EROFS only supports EROFS_INODE_FLAT_COMPRESSION_LEGACY
(per-filesystem setting), we'd like to introduce per-file setting and
more configration for future requirements....

>
> > > 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.

(Just my personal thought... erofs_inode_is_data_compressed operates
ondisk field like datamode (because we have 2 datamode for compression,
need to wrap them to judge if the file is compressed...)
so it stays at erofs_fs.h... is_inode_flat_inline operates in-memory
struct inode so it in internal.h....)

Thanks,
Gao Xiang

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