From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Williams Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t Date: Thu, 7 May 2015 13:06:52 -0700 Message-ID: References: <20150506200219.40425.74411.stgit@dwillia2-desk3.amr.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Linux Kernel Mailing List , Boaz Harrosh , Jan Kara , Mike Snitzer , Neil Brown , Benjamin Herrenschmidt , Dave Hansen , Heiko Carstens , Chris Mason , Paul Mackerras , "H. Peter Anvin" , Christoph Hellwig , Alasdair Kergon , "linux-nvdimm@lists.01.org" , Ingo Molnar , Mel Gorman , Matthew Wilcox , Ross Zwisler , Rik van Riel , Martin Schwidefsky , Jens Axboe , "Theodore Ts'o" , "Martin K. Petersen" , Juli To: Linus Torvalds Return-path: Received: from mail-wi0-f182.google.com ([209.85.212.182]:35613 "EHLO mail-wi0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751612AbbEGUGx (ORCPT ); Thu, 7 May 2015 16:06:53 -0400 Received: by widdi4 with SMTP id di4so4770608wid.0 for ; Thu, 07 May 2015 13:06:52 -0700 (PDT) In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, May 7, 2015 at 10:43 AM, Linus Torvalds wrote: > On Thu, May 7, 2015 at 9:03 AM, Dan Williams wrote: >> >> Ok, I'll keep thinking about this and come back when we have a better >> story about passing mmap'd persistent memory around in userspace. > > Ok. And if we do decide to go with your kind of "__pfn" type, I'd > probably prefer that we encode the type in the low bits of the word > rather than compare against PAGE_OFFSET. On some architectures > PAGE_OFFSET is zero (admittedly probably not ones you'd care about), > but even on x86 it's a *lot* cheaper to test the low bit than it is to > compare against a big constant. > > We know "struct page *" is supposed to be at least aligned to at least > "unsigned long", so you'd have two bits of type information (and we > could easily make it three). With "0" being a real pointer, so that > you can use the pointer itself without masking. > > And the "hide type in low bits of pointer" is something we've done > quite a lot, so it's more "kernel coding style" anyway. Ok. Although __pfn_t also stores pfn values directly which will consume those 2 bits so we'll need to shift pfns up when storing.