linux-kernel - Re: [PATCH] blk: missing add of padded bytes to io completion byte count

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Message-ID: <20080305123317.GI6704@kernel.dk>
Date:	Wed, 5 Mar 2008 13:33:18 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Tejun Heo <htejun@...il.com>, Mike Galbraith <efault@....de>,
	James.Bottomley@...senPartnership.com, tomof@....org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
	jgarzik@...ox.com, bzolnier@...il.com
Subject: Re: [PATCH] blk: missing add of padded bytes to io completion byte count

On Wed, Mar 05 2008, Boaz Harrosh wrote:
> On Wed, Mar 05 2008 at 2:26 +0200, FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:
> > On Wed, 05 Mar 2008 08:33:05 +0900
> > Tejun Heo <htejun@...il.com> wrote:
> > 
> >> FUJITA Tomonori wrote:
> >>> Hmm, does SCSI mid-layer need to care about how many bytes the block
> >>> layer allocates? I don't think that extra_len is NOT good_bytes.
> >>>
> >>> I think that the block layer had better take care about it (fix
> >>> __end_that_request_first?).
> >> Yeah, probably calling completion functions w/o bytes count is the right
> >> thing to do but what I was talking about was what could break when the
> >> semantics of rq->data_len changed.  If we keep rq->data_len() ==
> >> sum(sg), we keep it business as usual for all the rest except for the
> >> device application layer if we don't we do the reverse and SCSI midlayer
> >> completion was a good example, I think.
> > 
> > sglist is a low-level I/O representation for device drivers. SCSI
> > midlayer should not care about sglist. We should not fix SCSI midlayer
> > for rq->data_len != sum(sg) change (so I can't agree with your
> > diagrams in another mail).
> > 
> > When if we change a rule, we need to fix something.
> > 
> > If we keep rq->data_len == sum(sg), we need to fix the device
> > application layer. If we keep rq->data_len == the true data length, we
> > need to fix the low-level drivers.
> > 
> > Now I'm fine with the commit e97a294ef6938512b655b1abf17656cf2b26f709
> > since we are in -rc stages. But I plan to send a patch to revert it
> > and fix this issue in the block layer. I'd like to test it in -mm for
> > a while.
> 
> No this commit is a serious bug, and the only fix is like you suggested
> in __end_that_request_first. This is because it breaks that scsi-ml loop
> where scsi_bufflen() can be less then blk_rq_bytes(). In that case this 
> commit is a data corruption.
> 
> > Only sglist stuff in SCSI midlayer is scsi_req_map_sg now. As you
> > know, we really want to remove it.
> > 
> > 
> >> Things going the other way is fine with me but I at least want to hear a
> >> valid rationale.  Till now all I got is "because that's the true size"
> >> which doesn't really make much sense to me.
> > 
> > Most of users of request structure care about only the real data
> > length, don't care about padding and drain length. Why do they bother
> > to use a helper function to get the real data length?
> > --
> 
> Submitted is the right fix to this problem, as pointed out by TOMO.
> Please test it solves the CD burning problem.
> (The patch includes the revert of commit e97a294e)
> ---
> From: Boaz Harrosh <bharrosh@...asas.com>
> Date: Wed, 5 Mar 2008 12:07:12 +0200
> Subject: [PATCH] blk: missing add of padded bytes to io completion byte count
> 
> the commit e97a294ef6938512b655b1abf17656cf2b26f709 was very wrong. This is
> because scsi-ml supports the ability to split a request into smaller chunks,
> in which case scsi_bufflen() is smaller then request length. Then at completion
> time the remainder can be issued as a new scsi command. In that case the above
> commit is a data corruption.

We needed something for -rc4, so it had to be rushed a bit...

> Also in this fix all users of block layer are taken care of, and not only
> scsi devices.
> 
> Signed-off-by: Boaz Harrosh <bharrosh@...asas.com>
> Signed-off-by: Benny Halevy <bhalevy@...asas.com>
> ---
>  block/blk-core.c    |    4 ++++
>  drivers/scsi/scsi.c |    2 +-
>  2 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 2a438a9..37fcccc 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1549,6 +1549,9 @@ static int __end_that_request_first(struct request *req, int error,
>  			     nr_bytes >> 9, req->sector);
>  	}
>  
> +	if (nr_bytes >= blk_rq_bytes(req))
> +		nr_bytes += req->extra_len;
> +
>  	total_bytes = bio_nbytes = 0;
>  	while ((bio = req->bio) != NULL) {
>  		int nbytes;
> @@ -1616,6 +1619,7 @@ static int __end_that_request_first(struct request *req, int error,
>  	if (!req->bio)
>  		return 0;
>  
> +	BUG_ON(total_bytes >= blk_rq_bytes(req));

Make that a WARN_ON() first please. It's indeed a bug, but it wont be
critical and it's not fair killing everything since this padding stuff
is so fresh and may still need a tweak or two.

I'd be fine with making it a BUG_ON() post 2.6.25.

-- 
Jens Axboe

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