Re: [PATCH 1/3] pfn_t: Change the encoding

From: Dan Williams
Date: Fri Mar 11 2016 - 16:40:26 EST

On Fri, Mar 11, 2016 at 1:13 PM, Matthew Wilcox
<matthew.r.wilcox@xxxxxxxxx> wrote:
> By moving the flag bits to the bottom, we encourage commonality
> between SGs with pages and those using pfn_t. We can also then insert
> a pfn_t into a radix tree, as it uses the same two bits for indirect &
> exceptional indicators.

It's not immediately clear to me what we gain with SG entry
commonality. The down side is that we lose the property that
pfn_to_pfn_t() is a nop. This was Dave's suggestion so that the
nominal case did not change the binary layout of a typical pfn.

Can we just bit swizzle a pfn_t on insertion/retrieval from the radix?