linux-kernel - Re: [PATCH 1/7] block: Add block_flush

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Date:	Mon, 30 Mar 2009 17:07:41 +0200
From:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
To:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
Cc:	Jeff Garzik <jeff@...zik.org>,
	Christoph Hellwig <hch@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	chris.mason@...cle.com, david@...morbit.com, tj@...nel.org
Subject: Re: [PATCH 1/7] block: Add block_flush_device()

On Monday 30 March 2009, Fernando Luis Vázquez Cao wrote:
> This patch adds a helper function that should be used by filesystems that need
> to flush the underlying block device on fsync()/fdatasync().
> 
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@....ntt.co.jp>
> ---
> 
> diff -urNp linux-2.6.29-orig/fs/buffer.c linux-2.6.29/fs/buffer.c
> --- linux-2.6.29-orig/fs/buffer.c	2009-03-24 08:12:14.000000000 +0900
> +++ linux-2.6.29/fs/buffer.c	2009-03-30 15:27:04.000000000 +0900
> @@ -165,6 +165,17 @@ void end_buffer_write_sync(struct buffer
>   	put_bh(bh);
>   }
> 
> +/* Issue flush of write caches on the block device */
> +int block_flush_device(struct block_device *bdev)

I don't consider this an improvement over using blkdev_issue_flush().

> +{
> +	int ret = 0;
> +
> +	ret = blkdev_issue_flush(bdev, NULL);

The problem lies in using NULL for error_sector argument which shows
a subtle deficiency of the current implementation/usage of barriers
based on a write cache flushing.

I intend to document the issue with adding the FIXME to the current
users of blkdev_issue_flush() so the problem is at least known and not
forgotten (fixing it would require some work from both block and fs
sides and unfortunately there wasn't even a willingness to discuss
possible solutions few years back when the original code was added).

Thanks,
Bart
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