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

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Date:   Thu, 29 Aug 2019 02:59:54 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     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>,
        Christoph Hellwig <hch@...radead.org>,
        "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

> --- /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.

> +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.

> +/* 64 */__u8 volume_name[16];   /* volume name */
> +/* 80 */__le32 requirements;    /* (aka. feature_incompat) */
> +
> +/* 84 */__u8 reserved2[44];
> +} __packed;                     /* 128 bytes */

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.

> +/*
> + * erofs inode data mapping:
> + * 0 - inode plain without inline data A:
> + * inode, [xattrs], ... | ... | no-holed data
> + * 1 - inode VLE compression B (legacy):
> + * inode, [xattrs], extents ... | ...
> + * 2 - inode plain with inline data C:
> + * inode, [xattrs], last_inline_data, ... | ... | no-holed data
> + * 3 - inode compression D:
> + * inode, [xattrs], map_header, extents ... | ...
> + * 4~7 - reserved
> + */
> +enum {
> +	EROFS_INODE_FLAT_PLAIN,

This one doesn't actually seem to be used.

> +	EROFS_INODE_FLAT_COMPRESSION_LEGACY,

why are we adding a legacy field to a brand new file system?

> +	EROFS_INODE_FLAT_INLINE,
> +	EROFS_INODE_FLAT_COMPRESSION,
> +	EROFS_INODE_LAYOUT_MAX

It seems like these come from the on-disk format, in which case they
should have explicit values assigned to them.

Btw, I think it generally helps file system implementation quality
if you use a separate header for the on-disk structures vs in-memory
structures, as that keeps it clear in everyones mind what needs to
stay persistent and what can be chenged easily.

> +static bool erofs_inode_is_data_compressed(unsigned int datamode)
> +{
> +	if (datamode == EROFS_INODE_FLAT_COMPRESSION)
> +		return true;
> +	return datamode == EROFS_INODE_FLAT_COMPRESSION_LEGACY;
> +}

This looks like a really obsfucated way to write:

	return datamode == EROFS_INODE_FLAT_COMPRESSION ||
		datamode == EROFS_INODE_FLAT_COMPRESSION_LEGACY;

> +/* 28 */__le32 i_reserved2;
> +} __packed;

Sane comment as above.

> +
> +/* 32 bytes on-disk inode */
> +#define EROFS_INODE_LAYOUT_V1   0
> +/* 64 bytes on-disk inode */
> +#define EROFS_INODE_LAYOUT_V2   1
> +
> +struct erofs_inode_v2 {
> +/*  0 */__le16 i_advise;

Why do we have two inode version in a newly added file system?

> +#define ondisk_xattr_ibody_size(count)	({\
> +	u32 __count = le16_to_cpu(count); \
> +	((__count) == 0) ? 0 : \
> +	sizeof(struct erofs_xattr_ibody_header) + \
> +		sizeof(__u32) * ((__count) - 1); })

This would be much more readable as a function.

> +#define EROFS_XATTR_ENTRY_SIZE(entry) EROFS_XATTR_ALIGN( \
> +	sizeof(struct erofs_xattr_entry) + \
> +	(entry)->e_name_len + le16_to_cpu((entry)->e_value_size))

Same here.

> +/* available compression algorithm types */
> +enum {
> +	Z_EROFS_COMPRESSION_LZ4,
> +	Z_EROFS_COMPRESSION_MAX
> +};

Seems like an on-disk value again that should use explicitly assigned
numbers.