From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4AF75C433DB for ; Wed, 10 Mar 2021 14:27:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1975B64FF4 for ; Wed, 10 Mar 2021 14:27:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233097AbhCJO1T (ORCPT ); Wed, 10 Mar 2021 09:27:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59552 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230373AbhCJO0v (ORCPT ); Wed, 10 Mar 2021 09:26:51 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 29023C061760; Wed, 10 Mar 2021 06:26:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=fMRcAUQViDFbKSMCzio+nZwR3R7d5z+5tJ4oYqUVAyM=; b=DX3yw+7jm8vk9+eflfEViPntiM iSRvWHzc9BxqlUmr+6/Vkm+qr6xjowBVUFsKtF0EEGRn5WNMMNKA79ysVFg64GdBFelo3aUDgyq7p cGt+Z/bf01+Wl4HA0Bk1VjCX/q3YZs3OAVLSWY3XSqWG2a7L34LbeaMH3hHvdwodIvUfKxym4i9l7 umdcaXJUac96k4gdNzC2XXyI8murWFW7EfACp9WA3JFUGr+iogourTMBlCtmFEh7Uutlb6qXjRXlW vw2UZ/8FJ1hREoKp3KcvS6Z7hici2aTs/J5tJ6YKg1N8rongRZND9sgpPrZbkQDxKnX+sxE29HOrw 9PoWbUvg==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lJznP-003gjI-VB; Wed, 10 Mar 2021 14:26:45 +0000 Date: Wed, 10 Mar 2021 14:26:43 +0000 From: Matthew Wilcox To: Goldwyn Rodrigues Cc: Neal Gompa , Shiyang Ruan , linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-nvdimm@lists.01.org, linux-fsdevel , darrick.wong@oracle.com, dan.j.williams@intel.com, jack@suse.cz, viro@zeniv.linux.org.uk, Btrfs BTRFS , ocfs2-devel@oss.oracle.com, david@fromorbit.com, hch@lst.de Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax Message-ID: <20210310142643.GQ3479805@casper.infradead.org> References: <20210226002030.653855-1-ruansy.fnst@fujitsu.com> <20210310130227.GN3479805@casper.infradead.org> <20210310142159.kudk7q2ogp4yqn36@fiona> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210310142159.kudk7q2ogp4yqn36@fiona> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Mar 10, 2021 at 08:21:59AM -0600, Goldwyn Rodrigues wrote: > On 13:02 10/03, Matthew Wilcox wrote: > > On Wed, Mar 10, 2021 at 07:30:41AM -0500, Neal Gompa wrote: > > > Forgive my ignorance, but is there a reason why this isn't wired up to > > > Btrfs at the same time? It seems weird to me that adding a feature > > > > btrfs doesn't support DAX. only ext2, ext4, XFS and FUSE have DAX support. > > > > If you think about it, btrfs and DAX are diametrically opposite things. > > DAX is about giving raw access to the hardware. btrfs is about offering > > extra value (RAID, checksums, ...), none of which can be done if the > > filesystem isn't in the read/write path. > > > > That's why there's no DAX support in btrfs. If you want DAX, you have > > to give up all the features you like in btrfs. So you may as well use > > a different filesystem. > > DAX on btrfs has been attempted[1]. Of course, we could not But why? A completeness fetish? I don't understand why you decided to do this work. > have checksums or multi-device with it. However, got stuck on > associating a shared extent on the same page mapping: basically the > TODO above dax_associate_entry(). > > Shiyang has proposed a way to disassociate existing mapping, but I > don't think that is the best solution. DAX for CoW will not work until > we have a way of mapping a page to multiple inodes (page->mapping), > which will convert a 1-N inode-page mapping to M-N inode-page mapping. If you're still thinking in terms of pages, you're doing DAX wrong. DAX should work without a struct page.