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

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Message-ID: <20190830120714.GN2752@twin.jikos.cz>
Date:   Fri, 30 Aug 2019 14:07:14 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Joe Perches <joe@...ches.com>
Cc:     Gao Xiang <gaoxiang25@...wei.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Theodore Ts'o <tytso@....edu>, Pavel Machek <pavel@...x.de>,
        David Sterba <dsterba@...e.cz>,
        Amir Goldstein <amir73il@...il.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Jan Kara <jack@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, devel@...verdev.osuosl.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-erofs@...ts.ozlabs.org, Chao Yu <yuchao0@...wei.com>,
        Miao Xie <miaoxie@...wei.com>,
        Li Guifu <bluce.liguifu@...wei.com>,
        Fang Wei <fangwei1@...wei.com>
Subject: Re: [PATCH v6 01/24] erofs: add on-disk layout

On Thu, Aug 29, 2019 at 08:58:17AM -0700, Joe Perches wrote:
> On Thu, 2019-08-29 at 18:32 +0800, Gao Xiang wrote:
> > Hi Christoph,
> > 
> > On Thu, Aug 29, 2019 at 02:59:54AM -0700, Christoph Hellwig wrote:
> > > > --- /dev/null
> > > > +++ b/fs/erofs/erofs_fs.h
> > > > @@ -0,0 +1,316 @@
> > > > +/* SPDX-License-Identifier: GPL-2.0-only OR Apache-2.0 */
> > > > +/*
> > > > + * linux/fs/erofs/erofs_fs.h
> > > 
> > > Please remove the pointless file names in the comment headers.
> > 
> > Already removed in the latest version.
> > 
> > > > +struct erofs_super_block {
> > > > +/*  0 */__le32 magic;           /* in the little endian */
> > > > +/*  4 */__le32 checksum;        /* crc32c(super_block) */
> > > > +/*  8 */__le32 features;        /* (aka. feature_compat) */
> > > > +/* 12 */__u8 blkszbits;         /* support block_size == PAGE_SIZE only */
> > > 
> > > Please remove all the byte offset comments.  That is something that can
> > > easily be checked with gdb or pahole.
> > 
> > I have no idea the actual issue here.
> > It will help all developpers better add fields or calculate
> > these offsets in their mind, and with care.
> > 
> > Rather than they didn't run "gdb" or "pahole" and change it by mistake.
> 
> I think Christoph is not right here.
> 
> Using external tools for validation is extra work
> when necessary for understanding the code.

The advantage of using the external tools that the information about
offsets is provably correct ...

> The expected offset is somewhat valuable, but
> perhaps the form is a bit off given the visual
> run-in to the field types.
> 
> The extra work with this form is manipulating all
> the offsets whenever a structure change occurs.

... while this is error prone.

> The comments might be better with a form more like:
> 
> struct erofs_super_block {	/* offset description */
> 	__le32 magic;		/*   0  */
> 	__le32 checksum;	/*   4  crc32c(super_block) */
> 	__le32 features;	/*   8  (aka. feature_compat) */
> 	__u8 blkszbits;		/*  12  support block_size == PAGE_SIZE only */