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

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Message-ID: <1557307.tHJJu7g4Ig@wuerfel>
Date:	Fri, 12 Oct 2012 21:05:08 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	김재극 <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 Friday 05 October 2012 20:56:44 김재극 wrote:
> +struct f2fs_nat_entry {
> +       __u8 version;
> +       __le32 ino;
> +       __le32 block_addr;
> +} __packed;
> +
> +struct f2fs_nat_block {
> +       struct f2fs_nat_entry entries[NAT_ENTRY_PER_BLOCK];
> +} __packed;

Using "__packed" on structure is rather inefficient on CPU architectures
that cannot do aligned accesses. I would suggest you remove this
attribute everywhere you can. The f2fs_nat_entry is particularly
suboptimal because it is 9 bytes long, and I'm not sure if this
can be reasonably changed to a multiple of four.

In all other cases, you should try to lay out the structures
so that each member is naturally aligned and you don't need any __packed
attributes, in particular for those that are accessed a lot.

	Arnd
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