linux-kernel - Re: cdrom: use kmalloced buffers instead of buffers on stack

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Date:	Tue, 22 Apr 2008 20:47:01 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	tsbogend@...ha.franken.de
Cc:	jeff@...zik.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com, akpm@...ux-foundation.org
Subject: Re: cdrom: use kmalloced buffers instead of buffers on stack

On Tue, 22 Apr 2008 07:48:58 +0200
tsbogend@...ha.franken.de (Thomas Bogendoerfer) wrote:

> On Mon, Apr 21, 2008 at 10:01:26PM -0400, Jeff Garzik wrote:
> > >    cdrom: use kmalloced buffers instead of buffers on stack
> > >    
> > >    If cdrom commands are issued to a scsi drive in most cases the buffer 
> > >    will be
> > >    filled via dma.  This leads to bad stack corruption on non coherent 
> > >    platforms,
> > >    because the buffers are neither cache line aligned nor is the size a 
> > >    multiple
> > >    of the cache line size.  Using kmalloced buffers avoids this.
> > >    
> > >    Signed-off-by: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> > >    Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> > >---
> > > drivers/cdrom/cdrom.c |  274 
> > > ++++++++++++++++++++++++++++++++-----------------
> > > 1 files changed, 181 insertions(+), 93 deletions(-)
> > 
> > Eh... AFAICS this is only really useful in two of the cases converted.
> > 
> > For all the other cases (<= 32 bytes), it is _far_ less complex, far 
> > less code to simply communicate the additional alignment requirements to 
> > the compiler.
> > 
> > What about __attribute__ __aligned__?  Was that tried?
> 
> I used that while narrowing down the bug. But not only the alignment is
> important, but also size needs to be a multiple of the cache line size.
> Which means it needs to be 128 bytes for most SGI machines. That
> and the following in DMA-mapping.txt
> 
> "This rule also means that you may use neither kernel image addresses
> (items in data/text/bss segments), nor module image addresses, nor
> stack addresses for DMA."
> 
> let me choose the kmalloc() solution.

Can we advertise such architecture's dma restrictions? For example, if
we can update dma_pad_mask and dma_alignment in request_queue,
blk_rq_map_kern uses a proper bounce buffer for such
architectures. Then we can avoid putting extra complexity in uppper
drivers such as cdrom.c
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