Re: [PATCH 1/5] dax, pmem: Add a dax operation zero_page_range

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Feb 05, 2020 at 04:40:44PM -0800, Dan Williams wrote:
> On Wed, Feb 5, 2020 at 12:03 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Wed, Feb 05, 2020 at 10:30:50AM -0800, Christoph Hellwig wrote:
> > > > +   /*
> > > > +    * There are no users as of now. Once users are there, fix dm code
> > > > +    * to be able to split a long range across targets.
> > > > +    */
> > >
> > > This comment confused me.  I think this wants to say something like:
> > >
> > >       /*
> > >        * There are now callers that want to zero across a page boundary as of
> > >        * now.  Once there are users this check can be removed after the
> > >        * device mapper code has been updated to split ranges across targets.
> > >        */
> >
> > Yes, that's what I wanted to say but I missed one line. Thanks. Will fix
> > it.
> >
> > >
> > > > +static int pmem_dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
> > > > +                               unsigned int offset, size_t len)
> > > > +{
> > > > +   int rc = 0;
> > > > +   phys_addr_t phys_pos = pgoff * PAGE_SIZE + offset;
> > >
> > > Any reason not to pass a phys_addr_t in the calling convention for the
> > > method and maybe also for dax_zero_page_range itself?
> >
> > I don't have any reason not to pass phys_addr_t. If that sounds better,
> > will make changes.
> 
> The problem is device-mapper. That wants to use offset to route
> through the map to the leaf device. If it weren't for the firmware
> communication requirement you could do:
> 
> dax_direct_access(...)
> generic_dax_zero_page_range(...)
> 
> ...but as long as the firmware error clearing path is required I think
> we need to do pass the pgoff through the interface and do the pgoff to
> virt / phys translation inside the ops handler.

Hi Dan,

Drivers can easily convert offset into dax device (say phys_addr_t) to
pgoff and offset into page, isn't it?

pgoff_t = phys_addr_t/PAGE_SIZE;
offset = phys_addr_t & (PAGE_SIZE - 1);

And pgoff can easily be converted into sectors which dm/md can use for
mapping and come up with pgoff in target device.

Anyway, I am fine with anything.

Thanks
Vivek