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Subject: Re: 2.4.21-pre3 kernel crash
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jens Axboe <axboe@suse.de>
Cc: linux-kernel@vger.kernel.org,
       Larry Sendlosky <Larry.Sendlosky@storigen.com>
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	 <20030128220607.GF31566@suse.de> <1043828587.537.25.camel@zion.wanadoo.fr>
	 <20030129085332.GT31566@suse.de>
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Date: 29 Jan 2003 10:44:00 +0100
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On Wed, 2003-01-29 at 09:53, Jens Axboe wrote:
> On Wed, Jan 29 2003, Benjamin Herrenschmidt wrote:
> > On Tue, 2003-01-28 at 23:06, Jens Axboe wrote:
> > > On Tue, Jan 28 2003, Larry Sendlosky wrote:
> > > > I was glad to see the physical page support in 2.4.20.
> > > > (and also noticed that the current BK tree clobbered it
> > > > on a patch set from Alan).
> > > > 
> > > > One question, 
> > > > 
> > > > +		lastdataend = bh_phys(bh) + bh->b_size;
> > > > 
> > > > bh_phys(x) uses bh->b_page. Does it make a difference
> > > > if bh->b_page is zero? What if someone combines virt and phys
> > > > buffer addresses in bh list?
> > > 
> > > Yes good catch! New version attached.
> > 
> > That's interesting. I wasn't awaye you could have a request
> > containing such a "mixed" set of bh without valid pages.
> > Actually, I though b_page was always valid. Looking at
> > other drivers (typically the the csiss.c driver), it also
> > unconditionally use b_page & bh_phys(). So either we are
> > looking at a false problem, or that driver need fixing as
> > well.
> 
> b_page is not always valid for IDE, this is a special case. ide-scsi
> fabricates its own buffer_heads. cciss etc can rely on valid b_page
> always.

Ok. Well, looking at ide-scsi, I see that:

#if 1
			bh->b_data = sg->address;
#else
			if (sg->address) {
				bh->b_page = virt_to_page(sg->address);
				bh->b_data = (char *) ((unsigned long) sg->address & ~PAGE_MASK);
			} else if (sg->page) {
				bh->b_page = sg->page;
				bh->b_data = (char *) sg->offset;
			}
#endif

Can't we just turn that #if 1 into #if 0 ? The case of non segmented
requests remains, where we have

		bh->b_data = pc->scsi_cmd->request_buffer;
		bh->b_size = pc->request_transfer;

But then, can't we use the same approach as above using virt_to_page() ?

I don't say we should do it now. I'm mostly asking for my own eductation
about all this, though I admit I don't like the "mixing" done in
ide_build_sglist.

Ben.

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