Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752521AbbEGOmg (ORCPT ); Thu, 7 May 2015 10:42:36 -0400 Received: from mail-wi0-f169.google.com ([209.85.212.169]:33378 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752110AbbEGOmc (ORCPT ); Thu, 7 May 2015 10:42:32 -0400 Date: Thu, 7 May 2015 16:42:25 +0200 From: Ingo Molnar To: Dan Williams Cc: Linus Torvalds , 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" , Mel Gorman , Matthew Wilcox , Ross Zwisler , Rik van Riel , Martin Schwidefsky , Jens Axboe , "Theodore Ts'o" , "Martin K. Petersen" , Julia Lawall , Tejun Heo , linux-fsdevel , Andrew Morton Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t Message-ID: <20150507144225.GA20491@gmail.com> References: <20150506200219.40425.74411.stgit@dwillia2-desk3.amr.corp.intel.com> <20150507090217.GA4467@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150507090217.GA4467@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2298 Lines: 58 * Ingo Molnar wrote: > [...] > > For anything more complex, that maps any of this storage to > user-space, or exposes it to higher level struct page based APIs, > etc., where references matter and it's more of a cache with > potentially multiple users, not an IO space, the natural API is > struct page. Let me walk back on this: > I'd say that this particular series mostly addresses the 'pfn as > sector_t' side of the equation, where persistent memory is IO space, > not memory space, and as such it is the more natural and thus also > the cheaper/faster approach. ... but that does not appear to be the case: this series replaces a 'struct page' interface with a pure pfn interface for the express purpose of being able to DMA to/from 'memory areas' that are not struct page backed. > Linus probably disagrees? :-) [ and he'd disagree rightfully ;-) ] So what this patch set tries to achieve is (sector_t -> sector_t) IO between storage devices (i.e. a rare and somewhat weird usecase), and does it by squeezing one device's storage address into our formerly struct page backed descriptor, via a pfn. That looks like a layering violation and a mistake to me. If we want to do direct (sector_t -> sector_t) IO, with no serialization worries, it should have its own (simple) API - which things like hierarchical RAID or RDMA APIs could use. If what we want to do is to support say an mmap() of a file on persistent storage, and then read() into that file from another device via DMA, then I think we should have allocated struct page backing at mmap() time already, and all regular syscall APIs would 'just work' from that point on - far above what page-less, pfn-based APIs can do. The temporary struct page backing can then be freed at munmap() time. And if the usage is pure fd based, we don't really have fd-to-fd APIs beyond the rarely used splice variants (and even those don't do pure cross-IO, they use a pipe as an intermediary), so there's no problem to solve I suspect. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/