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Date: Thu, 13 Aug 2015 13:35:52 -0400
From: Matthew Wilcox <willy@linux.intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-kernel@vger.kernel.org, axboe@kernel.dk, riel@redhat.com,
        linux-nvdimm@ml01.01.org, linux-mm@kvack.org, mgorman@suse.de,
        torvalds@linux-foundation.org, hch@lst.de
Subject: Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into KVA
Message-ID: <20150813173552.GA9645@linux.intel.com>
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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).