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

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Date:	Tue, 31 Mar 2009 09:57:18 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Ric Wheeler <rwheeler@...hat.com>,
	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Jeff Garzik <jeff@...zik.org>,
	Christoph Hellwig <hch@...radead.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 Tue, 31 Mar 2009, Jens Axboe wrote:
> 
> So here's a test patch that attempts to just ignore such a failure to
> flush the caches.

I suspect you should not do it like this.

> diff --git a/fs/bio.c b/fs/bio.c
> index a040cde..79e3cec 100644
> --- a/fs/bio.c
> +++ b/fs/bio.c
> @@ -1380,7 +1380,17 @@ void bio_check_pages_dirty(struct bio *bio)
>   **/
>  void bio_endio(struct bio *bio, int error)
>  {
> -	if (error)
> +	/*
> +	 * Special case here - hide the -EOPNOTSUPP from the driver or
> +	 * block layer, dump a warning the first time this happens so that
> +	 * the admin knows that we may not provide the ordering guarantees
> +	 * that are needed. Don't clear the uptodate bit.
> +	 */
> +	if (error == -EOPNOTSUPP && bio_barrier(bio)) {
> +		set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
> +		blk_queue_set_noflush(bio->bi_bdev);
> +		error = 0;
> +	} else if (error)

I suspect this part is just wrong.

I could easily imagine a driver that returns EOPNOTSUPP only for a certain 
_kind_ of bio.

For example, if the drive doesn't support FUA, then you cannot do a 
serialized IO operation, but you can still mostly do a serialized op 
without any IO attached to it.

IOW, the "empty flush" really _is_ special. An this check should not be in 
the generic "bio_endio()" case, it should only be in the special 
blkdev_issue_flush() case.

I think. No?

		Linus
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