RFC: prepare for struct scatterlist entries without page backing

[Julian Calaby]

Hi Christoph,

On Fri, Aug 14, 2015 at 12:35 AM, Christoph Hellwig <hch at lst.de> wrote:
> 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.

I was talking in terms of changes made, not lines changed: by my
recollection, about a third of the patches didn't touch flush calls
and most of the lines changed looked like refactoring so that making
the flush call conditional would be easier.

I guess it smelled like you were doing lots of distinct changes in a
single patch and I got my numbers wrong.

>> 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.

My point is that there's a couple of patches that only remove
BUG_ON()s, which implies that for that particular driver it doesn't
matter if there's a physical page or not, so therefore that code is
purely "documentation".

Thanks,

-- 
Julian Calaby

Email: julian.calaby at gmail.com
Profile: http://www.google.com/profiles/julian.calaby/