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

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Date:	Fri, 11 Mar 2016 13:40:20 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc:	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 1/3] pfn_t: Change the encoding

On Fri, Mar 11, 2016 at 1:13 PM, Matthew Wilcox
<matthew.r.wilcox@...el.com> 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?