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

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Message-ID: <20190902130245.GC17916@architecture4>
Date:   Mon, 2 Sep 2019 21:02:45 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Gao Xiang <hsiangkao@....com>, Jan Kara <jack@...e.cz>,
        Dave Chinner <david@...morbit.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Miao Xie <miaoxie@...wei.com>, <devel@...verdev.osuosl.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Theodore Ts'o <tytso@....edu>, Pavel Machek <pavel@...x.de>,
        David Sterba <dsterba@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-erofs@...ts.ozlabs.org>
Subject: Re: [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