RFC: prepare for struct scatterlist entries without page backing

[Christoph Hellwig]

On Thu, Aug 13, 2015 at 09:37:37AM +1000, Julian Calaby wrote:
> I.e. ~90% of this patch set seems to be just mechanically dropping
> BUG_ON()s and converting open coded stuff to use accessor functions
> (which should be macros or get inlined, right?) - and the remaining
> bit is not flushing if we don't have a physical page somewhere.

Which is was 90%.  By lines changed most actually is the diffs for
the cache flushing.

> Would it make sense to split this patch set into a few bits: one to
> drop all the useless BUG_ON()s, one to convert all the open coded
> stuff to accessor functions, then another to do the actual page-less
> sg stuff?

Without the ifs the BUG_ON() actually are useful to assert we
never feed the sort of physical addresses we can't otherwise support,
so I don't think that part is doable.

A simple series to make more use of sg_phys and add sg_pfn might
still be useful, though.