Re: pvSCSI question - can a gref describe more than one page?

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 03/09/2010 07:18 PM, James Harper wrote:
> Porting the pvSCSI backend driver to the pvops kernel is turning out to
> be a bit harder than first thought, mostly because all the Linux
> routines that might be useful are written with the assumption that they
> would be passed a mapped buffer and they would then build an sg list out
> of that. pvSCSI already has an sg list, and all the routines that it
> makes use of have been deprecated since 2.6.18 (all the comments on the
> patches that remove the routines are like "nobody uses this anymore".
> D'oh.)
>
> The big complication appears to be that pvSCSI seems to think that an
> individual sg element can span a page, and so has to muck around
> splitting it into page-sized chunks. My assumption is that this is
> because the code in pvSCSI was cut&pasted from somewhere else.
>    

In general its a bug if an sg entry crosses a page boundary.  We define 
BIOVEC_PHYS_MERGEABLE so that we avoid doing any cross-page merges 
(unless, I think, the underlying mfns are actually contiguous).  Does 
that help?

     J