linux-kernel - Re: [bug] block subsystem related crash with latest -git

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Message-ID: <20071017182013.GR15552@kernel.dk>
Date:	Wed, 17 Oct 2007 20:20:25 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [bug] block subsystem related crash with latest -git

On Wed, Oct 17 2007, Linus Torvalds wrote:
> 
> 
> On Wed, 17 Oct 2007, Jens Axboe wrote:
> > >
> > >  - remove the "memset()" you had added earlier. It's bogus. It cannot be 
> > >    the right thing. If the sg list wasn't initialized correctly much 
> > >    earlier, trying to initialize it late is pointless - it contains crap.
> > 
> > It's required to clear output members (like dma_len and so on), since
> > some of the IOMU code really wants that initialized.
> 
> No it's NOT!
> 
> The whole point here is that the sg had *already* better be cleared.
> 
> If it wasn't cleared before, that's a bug regardless of anything else. So 
> a memset() is guaranteed to be either a no-op or hiding another bug!
> 
> So yes, those members had better be zero. It's just that they had better 
> be zero long before that memset! The memset should have been done when 
> allocating the SG list.

For the chain elements - yes, definitely! But we also want to clear dma
mapping output values, at least sparc64 wants that. You could argue that
the IOMMU code should be fixed up, but I don't think we should mix the
two.

So we need the memset() in blk_rq_map_sg() AS WELL AS the initial sg
table clear. I'm not arguing about the latter, we clearly do need that.

> > If you prefer the old next_sg approach to my alternative, that is fine
> > with me. But we do need the memset().
> 
> Really? Explain why. Entering that code with a non-initialized SG list is 
> a bug to begin with.

Not for the output members, they will be filled AFTER blk_rq_map_sg()
returns and someone use iommu_map_sg() to re-iterate the sglist and map
the pages appropriately.

-- 
Jens Axboe

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