Re: [PATCH v3 04/14] mm/memremap: add ZONE_DEVICE support for compound pages

[Joao Martins]

On 7/15/21 1:59 PM, Christoph Hellwig wrote:
> On Wed, Jul 14, 2021 at 06:08:14PM -0700, Dan Williams wrote:
>>> +static inline unsigned long pgmap_geometry(struct dev_pagemap *pgmap)
>>> +{
>>> +       if (!pgmap || !pgmap->geometry)
>>> +               return PAGE_SIZE;
>>> +       return pgmap->geometry;
>>> +}
>>> +
>>> +static inline unsigned long pgmap_pfn_geometry(struct dev_pagemap *pgmap)
>>> +{
>>> +       return PHYS_PFN(pgmap_geometry(pgmap));
>>> +}
>>
>> Are both needed? Maybe just have ->geometry natively be in nr_pages
>> units directly, because pgmap_pfn_geometry() makes it confusing
>> whether it's a geometry of the pfn or the geometry of the pgmap.
> 
> Actually - do we need non-power of two sizes here?  Otherwise a shift
> for the pfns would be really nice as that simplifies a lot of the
> calculations.
> 
AIUI, it's only powers-of-two: PAGE_SIZE (1 if nr of pages), PMD_SIZE (512) and 
PUD_SIZE (4K).