How to map high memory for block io

[Pierre Ossman <drzeus-list@drzeus.cx>]

I'm having some problems getting high memory support to work smoothly in
my driver. The documentation doesn't indicate what I might be doing
wrong so I'll have to ask here.

The problem seems to be that kmap & co maps a single page into kernel
memory. So when I happen to cross page boundaries I start corrupting
some unrelated parts of the kernel. I would prefer not having to
consider page boundaries in an already messy PIO loop, so I've been
trying to find either a routine to map an entire sg entry or some way to
force the block layer to not give me stuff crossing pages.

As you can guess I have not found anything that can do what I want, so
some pointers would be nice.

Rgds
Pierre