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Date:   Mon, 2 Sep 2019 10:40:20 +0200
From:   Pavel Machek <pavel@denx.de>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Gao Xiang <gaoxiang25@huawei.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Theodore Ts'o <tytso@mit.edu>, Pavel Machek <pavel@denx.de>,
        David Sterba <dsterba@suse.cz>,
        Amir Goldstein <amir73il@gmail.com>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        Dave Chinner <david@fromorbit.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Jan Kara <jack@suse.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, devel@driverdev.osuosl.org,
        LKML <linux-kernel@vger.kernel.org>,
        linux-erofs@lists.ozlabs.org, Chao Yu <yuchao0@huawei.com>,
        Miao Xie <miaoxie@huawei.com>,
        Li Guifu <bluce.liguifu@huawei.com>,
        Fang Wei <fangwei1@huawei.com>
Subject: Re: [PATCH v6 01/24] erofs: add on-disk layout
Message-ID: <20190902084020.GB19557@amd>
References: <20190802125347.166018-1-gaoxiang25@huawei.com>
 <20190802125347.166018-2-gaoxiang25@huawei.com>
 <20190829095954.GB20598@infradead.org>
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Hi!

> > +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 =3D=3D PAGE_SIZE=
 only */
>=20
> Please remove all the byte offset comments.  That is something that can
> easily be checked with gdb or pahole.

I don't think I agree. gdb will tell you byte offsets _on one
architecture_. But filesystem is supposed to be portable between them.=20

> > +/* 64 */__u8 volume_name[16];   /* volume name */
> > +/* 80 */__le32 requirements;    /* (aka. feature_incompat) */
> > +
> > +/* 84 */__u8 reserved2[44];
> > +} __packed;                     /* 128 bytes */
>=20
> Please don't add __packed.  In this case I think you don't need it
> (but double check with pahole), but even if you would need it using
> proper padding fields and making sure all fields are naturally aligned
> will give you much better code generation on architectures that don't
> support native unaligned access.

This is on-disk structure, right?

drivers/staging/erofs/super.c:	struct erofs_super_block *layout;
drivers/staging/erofs/super.c:	layout =3D (struct erofs_super_block
*)((u8 *)bh->b_data

So __packed is right thing to do. If architecture accesses that
slowly, that's ungood, but different structures between architectures
would be really bad.

Best regards,
								Pavel
--=20
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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