From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753391AbcCLS3w (ORCPT ); Sat, 12 Mar 2016 13:29:52 -0500 Received: from mga04.intel.com ([192.55.52.120]:25720 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752249AbcCLS3o (ORCPT ); Sat, 12 Mar 2016 13:29:44 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.24,326,1455004800"; d="scan'208";a="922577015" Date: Sat, 12 Mar 2016 13:30:05 -0500 From: Matthew Wilcox To: Dan Williams Cc: Matthew Wilcox , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" , Linux MM , Dave Hansen Subject: Re: [PATCH 1/3] pfn_t: Change the encoding Message-ID: <20160312183005.GA2525@linux.intel.com> References: <1457730784-9890-1-git-send-email-matthew.r.wilcox@intel.com> <1457730784-9890-2-git-send-email-matthew.r.wilcox@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 11, 2016 at 01:40:20PM -0800, Dan Williams wrote: > On Fri, Mar 11, 2016 at 1:13 PM, Matthew Wilcox > 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. I understand that motivation! > Can we just bit swizzle a pfn_t on insertion/retrieval from the radix? Of course we *can*, but we end up doing more swizzling that way than we do this way. In the Brave New Future where we're storing pfn_t in the radix tree, on a page fault we find the pfn_t in the radix tree then we want to insert it into the page tables. So DAX would first have to convert the radix tree entry to a pfn_t, then the page table code has to convert the pfn_t into a pte/pmd/pud (which we currently do by converting a pfn_t to a pfn, then converting the pfn to a pte/pmd/pud, but I assume that either the compiler optimises that into a single conversion, or we'll add pfn_t_pte to each architecture in future if it's actually a problem). Much easier to look up a pfn_t in the radix tree and pass it directly to vm_insert_mixed(). If there's any part of the kernel that is doing a *lot* of conversion between pfn_t and pfn, that surely indicates a place in the kernel where we need to convert an interface from pfn to pfn_t. (It occurs to me we can make the code simpler on architectures that don't support PUDs. The PFN_HUGE bit is still available to distinguish between PMDs and PTEs, but we won't need to clear the bottom bit of the PFN if PFN_HUGE is set, since nobody can add a PUD pfn to the radix tree).