Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into KVA

[Matthew Wilcox <willy@linux.intel.com>]

On Wed, Aug 12, 2015 at 11:01:09PM -0400, Dan Williams wrote:
> +static inline __pfn_t page_to_pfn_t(struct page *page)
> +{
> +	__pfn_t pfn = { .val = page_to_pfn(page) << PAGE_SHIFT, };
> +
> +	return pfn;
> +}

static inline __pfn_t page_to_pfn_t(struct page *page)
{
	__pfn_t __pfn;
	unsigned long pfn = page_to_pfn(page);
	BUG_ON(pfn > (-1UL >> PFN_SHIFT))
	__pfn.val = pfn << PFN_SHIFT;

	return __pfn;
}

I have a problem wih PFN_SHIFT being equal to PAGE_SHIFT.  Consider a
32-bit kernel; you're asserting that no memory represented by a struct
page can have a physical address above 4GB.

You only need three bits for flags so far ... how about making PFN_SHIFT
be 6?  That supports physical addresses up to 2^38 (256GB).  That should
be enough, but hardware designers have done some strange things in the
past (I know that HP made PA-RISC hardware that can run 32-bit kernels
with memory between 64GB and 68GB, and they can't be the only strange
hardware people out there).