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

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Message-ID: <49D1B3B7.9000109@oss.ntt.co.jp>
Date:	Tue, 31 Mar 2009 15:09:59 +0900
From:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
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()

Bartlomiej Zolnierkiewicz wrote:
> 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().

The reason I used a wrapper is that I did not like the semantics provided
by blkdev_issue_flush(). On the one hand, I did not want to pass -EOPNOTSUPP
to filesystems (it is not an error filesystems should care about). On the
other hand it is weird that some filesystems use blkdev_issue_flush() when
they want emit a barrier. blkdev_issue_flush() happens to be implemented
as an empty (block layer) barrier, but I think that is an implementation
detail filesystems should not neet to know about. Indeed I am working on a
patch that implements blkdev_issue_empty_barrier(), so that we can optimize
fsync() flushes and filesystem-originated barriers independently in the block
layer.

Judging from your comments below, it seems we are in the same page regarding
this issue.

Again, thank you for you feedback!

- Fernando

>> +{
>> +	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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