Date: Thu, 10 May 2007 23:37:43 +0200
From: Jens Axboe <jens.axboe@oracle.com>
To: Benny Halevy <bhalevy@panasas.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/12] crypto: don't pollute the global namespace with  sg_next()
Message-ID: <20070510213743.GY4163@kernel.dk>
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On Thu, May 10 2007, Benny Halevy wrote:
> Jens Axboe wrote:
> > It's a subsystem function, prefix it as such.
> 
> Jens, Boaz and I talked about this over lunch.
> I wonder whether the crypto code must use your implementation
> instead of its own as it needs to over the sglist, e.g. for
> calculating iscsi (data) digest.

The thought did cross my mind, and yes I think that would be a good
idea. The whole thing should probably just migrate to
lib/scattersomething.c

> The crypto implementation of chained sglists in crypto/scatterwalk.h
> determines the chain link by !sg->length which will sorta work
> with your implementation, however the marker bit on page pointer must
> be cleared to use it.

I don't like using sg->length, as that may be modified for legitimate
reason. That's why I chose to use the lsb bit of the page pointer.

> Also, is it possible that after the original sglist has gone through
> dma_map_sg and entries were merged, some entries will have zero
> length?  I'm not sure... If so, if the crypto implementation scans
> the sg list after it was dma mapped (maybe in a retry path) it
> may hit an entry that looks to it like a chaining link.  This
> might be an existing bug and another reason for the crypto code
> to use your implementation.

It's hard to say, depends heavily on the sub system or arch. Even if
using the pointer tagging mechanism seems a bit nasty, I think it's the
more resilient approach.

-- 
Jens Axboe

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