linux-kernel - Re: [PATCH] block: fix residual byte count handling

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Date:	Fri, 29 Feb 2008 21:11:25 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Tejun Heo <htejun@...il.com>, Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-ide@...r.kernel.org,
	linux-scsi@...r.kernel.org, Jeff Garzik <jgarzik@...ox.com>
Subject: Re: [PATCH] block: fix residual byte count handling

On Fri, Feb 29 2008, James Bottomley wrote:
> 
> On Fri, 2008-02-29 at 00:46 +0900, Tejun Heo wrote:
> > Jens Axboe wrote:
> > >> This problem was reported and diagnosed by Mike Galbraith.
> > > 
> > > Tejun, this patch isn't much cleaner at all. It really shows the pain of
> > > these two seperate, yet related, variables.
> > 
> > Not much cleaner compared to what?  I think padding stuff is bound to be
> > somewhat complex.  It's a nasty thing in nature.  I think ->extra_len is
> > better than ->raw_data_len because ->extra_len only needs to be updated
> > where the dirty jobs are done and extra buffer areas are added.  Any
> > better suggestions?
> 
> Well, I just investigated a bug report in the SCSI transport class.  Our
> SMP handler is broken in exactly the same way.  We rely on the incoming
> reported request lengths to size our request data, and they've blown up
> from the true length to 512 bytes (the size of our alignment).
> 
> With the original patch, I have to run through the whole of libsas and
> scsi_transport_sas doing
> 
> s/data_len/raw_data_len/
> 
> With your update it looks like I have to run through them all doing
> 
> s/data_len/data_len - extra_len/
> 
> which is even worse.  Can't we put things back to a point where data_len
> means exactly that and extra_len means how much we have spare on the
> end, so you know you can DMA up to data_len + extra_len if need be?
> 
> That way we don't have to sweep through every block driver altering the
> way it uses data_len.

Fully agree. The reason why I think it's so ugly is that you have to
keep these two seperate variables in sync. The burning was just one bug,
there will be others...

-- 
Jens Axboe

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