Date: Fri, 29 Feb 2008 21:11:25 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Tejun Heo <htejun@gmail.com>, Mike Galbraith <efault@gmx.de>,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>, linux-ide@vger.kernel.org,
       linux-scsi@vger.kernel.org, Jeff Garzik <jgarzik@pobox.com>
Subject: Re: [PATCH] block: fix residual byte count handling
Message-ID: <20080229201123.GK6704@kernel.dk>
References: <20080226150845.2196bc1a.akpm@linux-foundation.org> <1204079075.26640.8.camel@homer.simson.net> <1204092010.9934.31.camel@homer.simson.net> <1204096025.1623.9.camel@homer.simson.net> <47C6661E.9010504@gmail.com> <1204186800.7362.7.camel@homer.simson.net> <47C675C6.8000904@gmail.com> <20080228153542.GZ6704@kernel.dk> <47C6D73E.4030302@gmail.com> <1204303656.4003.19.camel@localhost.localdomain>
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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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