Re: [PATCH] "killing" sg_last(), and discussion

[Boaz Harrosh <bharrosh@panasas.com>]

On Wed, Oct 31 2007 at 10:49 +0200, Jeff Garzik <jeff@garzik.org> wrote:
> I looked into killing sg_last(), but really, this is the best its gonna
> get (moving sg_last to libata-core.c).
> 
> You could maybe kill one use with caching, but in the other sg_last()
> callsites there isn't another s/g loop we can stick a "last_sg = sg;"
> into.
> 
> libata is stuck because we undertake the highly unusual operation of
> fiddling with the final S/G element, to enforce 32-bit alignment.
> 
> Of course we could eliminate all that nasty fiddling/padding
> completely, including sg_last(), if other areas of the kernel would
> guarantee ahead of time that buffer lengths are always a multiple
> of 4........
> 
> 	Jeff
> 
OK Now I'm confused. I thought that ULD's can give you SG's 
that are actually longer than bufflen and that, at the end, the 
bufflen should govern the transfer length.

Now FS_PC commands are sector aligned so you do not have
problems with that.

The BLOCK_PC commands have 2 main sources that I know of
one is sg && bsg from user mode that can easily enforce
4 bytes alignment. The second is kernel services which 80%
of these are done by scsi_execute(). All These can be found
and fixed. Starting with scsi_execute(). Another place can be
blk_rq_map_sg(), since all IO's are bio based. It can enforce 
alignment too.

I would start by sticking a WARN_ON(qc->pad_len) and
see if it triggers, what are the sources of that.

Please note that the code already has a 4 bytes alignment
assumption about the start of the transfer, other wise
the first SG can also have a none aligned length, which
is not checked for.

Boaz
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