# Re: \[PATCH v1 04/11\] mm/memremap: add ZONE\_DEVICE support for compound pages

- From: Matthew Wilcox \<willy@infradead.org\>
- Date: 2021-05-05 18:59:55 
- List: linux-mm
- Page: https://ratatoskr.run/linux-mm/2021/05/6507971
- Thread: https://ratatoskr.run/linux-mm/2021/03/6507171/t.md

```text
On Wed, May 05, 2021 at 11:44:29AM -0700, Dan Williams wrote:
> > @@ -6285,6 +6285,8 @@ void __ref memmap_init_zone_device(struct zone *zone,
> >         unsigned long pfn, end_pfn = start_pfn + nr_pages;
> >         struct pglist_data *pgdat = zone->zone_pgdat;
> >         struct vmem_altmap *altmap = pgmap_altmap(pgmap);
> > +       unsigned int pfn_align = pgmap_pfn_align(pgmap);
> > +       unsigned int order_align = order_base_2(pfn_align);
> >         unsigned long zone_idx = zone_idx(zone);
> >         unsigned long start = jiffies;
> >         int nid = pgdat->node_id;
> > @@ -6302,10 +6304,30 @@ void __ref memmap_init_zone_device(struct zone *zone,
> >                 nr_pages = end_pfn - start_pfn;
> >         }
> >
> > -       for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> > +       for (pfn = start_pfn; pfn < end_pfn; pfn += pfn_align) {
> 
> pfn_align is in bytes and pfn is in pages... is there a "pfn_align >>=
> PAGE_SHIFT" I missed somewhere?

If something is measured in bytes, I like to use size_t (if it's
in memory) and loff_t (if it's on storage).  The compiler doesn't do
anything useful to warn you, but it's a nice indication to humans about
what's going on.  And it removes the temptation to do 'pfn_align >>=
PAGE_SHIFT' and suddenly take pfn_align from being measured in bytes to
being measured in pages.


```