Re: [PATCH v3 10/25] fsdax: Introduce pgmap_request_folios()

[Dan Williams <dan.j.williams@intel.com>]

Jason Gunthorpe wrote:
> On Mon, Oct 17, 2022 at 01:51:18PM -0700, Dan Williams wrote:
> 
> > > A good design is that nothing has a struct folio * unless it can also
> > > prove it has a non-zero refcount on it, and that is simply impossible
> > > for any caller at this point.
> > 
> > I agree that's a good design, and I think it is a bug to make this
> > request on anything but a zero-refcount folio, but plumbing pgmap
> > offsets to replace pfn_to_page() would require dax_direct_access() to
> > shift from a pfn lookup to a pgmap + offset lookup.
> 
> That is fair for this series, but maybe the other pgmap users shouldn't be
> converted to use this style of API?

That is also fair...

I think my main stumbling block is getting back to the pgmap from the
DAX code. However, I could move that to a dirty-hack that looks it up
via dax_direct_access()+pfn_to_page(), but keep the core request API
based on pgmap-offset.

I.e. it needs some access to a 0-referenced page, but it moves that wart
out-of-line of the main flow. Some of the non-DAX users might need this
hack as well unless I can figure out how to route back to the pgmap
without interrogating the pfn.

I'll try it incrementally to the current series to make things easier
for Andrew.