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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 0183FC43331 for ; Thu, 5 Sep 2019 22:44:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 46D9320825 for ; Thu, 5 Sep 2019 22:44:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728590AbfIEWoI (ORCPT ); Thu, 5 Sep 2019 18:44:08 -0400 Received: from mail105.syd.optusnet.com.au ([211.29.132.249]:48353 "EHLO mail105.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725290AbfIEWoI (ORCPT ); Thu, 5 Sep 2019 18:44:08 -0400 Received: from dread.disaster.area (pa49-181-255-194.pa.nsw.optusnet.com.au [49.181.255.194]) by mail105.syd.optusnet.com.au (Postfix) with ESMTPS id 2D13836142E; Fri, 6 Sep 2019 08:44:05 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92) (envelope-from ) id 1i60Tz-00031P-Qh; Fri, 06 Sep 2019 08:44:03 +1000 Date: Fri, 6 Sep 2019 08:44:03 +1000 From: Dave Chinner To: David Howells Cc: Christoph Hellwig , Carlos Maiolino , linux-fsdevel@vger.kernel.org, adilger@dilger.ca, jaegeuk@kernel.org, darrick.wong@oracle.com, miklos@szeredi.hu, rpeterso@redhat.com, linux-xfs@vger.kernel.org Subject: Re: [PATCH 2/9] cachefiles: drop direct usage of ->bmap method. Message-ID: <20190905224403.GK1119@dread.disaster.area> References: <20190830165920.GA28182@lst.de> <20190829071312.GE11909@lst.de> <20190820115731.bed7gwfygk66nj43@pegasus.maiolino.io> <20190808082744.31405-1-cmaiolino@redhat.com> <20190808082744.31405-3-cmaiolino@redhat.com> <20190814111535.GC1885@lst.de> <7003.1566305430@warthog.procyon.org.uk> <2587.1567181871@warthog.procyon.org.uk> <28402.1567212357@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <28402.1567212357@warthog.procyon.org.uk> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.2 cv=D+Q3ErZj c=1 sm=1 tr=0 a=YO9NNpcXwc8z/SaoS+iAiA==:117 a=YO9NNpcXwc8z/SaoS+iAiA==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=kj9zAlcOel0A:10 a=J70Eh1EUuV4A:10 a=7-415B0cAAAA:8 a=yx1zXdAvrB0BR2aM9G4A:9 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Sat, Aug 31, 2019 at 01:45:57AM +0100, David Howells wrote: > Christoph Hellwig wrote: > > > We'll you'd need to implement a IOCB_NOHOLE, but that wouldn't be all > > that hard. Having a READ_PLUS like interface that actually tells you > > how large the hole is might be hard. > > Actually, that raises another couple of questions: > > (1) Is a filesystem allowed to join up two disjoint blocks of data with a > block of zeros to make a single extent? If this happens, I'll see the > inserted block of zeros to be valid data. Yes. > (2) Is a filesystem allowed to punch out a block of valid zero data to make a > hole? This would cause me to refetch the data. Yes. Essentially, assumptions that filesystems will not change the file layout even if the data does not change are invalid. copy-on-write, deduplication, speculative preallocation, etc mean the file layout can change even if the file data itself is not directly modified. If you want to cache physical file layout information and be notified when they may change, then that's what layout leases are for.... Cheers, Dave. -- Dave Chinner david@fromorbit.com