linux-kernel - Re: [PATCH 02/16] f2fs: add on-disk layout

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Date:	Sun, 7 Oct 2012 00:21:05 +0200
From:	David Sterba <dave@...os.cz>
To:	????????? <jaegeuk.kim@...sung.com>
Cc:	viro@...iv.linux.org.uk, "'Theodore Ts'o'" <tytso@....edu>,
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	chur.lee@...sung.com, cm224.lee@...sung.com,
	jooyoung.hwang@...sung.com
Subject: Re: [PATCH 02/16] f2fs: add on-disk layout

On Fri, Oct 05, 2012 at 08:56:44PM +0900, ????????? wrote:
> +struct node_footer {
> +	__le32 nid;		/* node id */
> +	__le32 ino;		/* inode nunmber */
> +	__le32 cold:1;		/* cold mark */
> +	__le32 fsync:1;		/* fsync mark */
> +	__le32 dentry:1;	/* dentry mark */
> +	__le32 offset:29;	/* offset in inode's node space */

A bitfield for a on-disk structure? This will have endianity issues,
(but I don't know if you intend to support big-endian). It's not enough
to use cpu_to_le* as in

fill_node_footer(...) {

	rn->footer.offset = cpu_to_le32(ofs);

}

because the bitfield inside the structure will be already defined
reversed. The cpu_to_le macro will only convert value of 'ofs' but will
place it to different bits than it would on a little-endian arch.

There are macros to define bitfields in an endian-neutral way (or do it
by #ifdefs though it also involves duplicating the item names), or you
can alternatively use two structs fr disk-only and memory-only access,
the disk one stores __le32 with value combined of all and the in-memory
gets set up properly and will look like your current version of the
structure.

(More about not using bitfields http://yarchive.net/comp/linux/bitfields.html)

> +	__le64 cp_ver;		/* checkpoint version */
> +	__le32 next_blkaddr;	/* next node page block address */
> +} __packed;
> +

david
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