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=-7.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 CE6B8C33CB1 for ; Wed, 15 Jan 2020 08:39:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AED4E2187F for ; Wed, 15 Jan 2020 08:39:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729014AbgAOIi6 (ORCPT ); Wed, 15 Jan 2020 03:38:58 -0500 Received: from verein.lst.de ([213.95.11.211]:49605 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726513AbgAOIi6 (ORCPT ); Wed, 15 Jan 2020 03:38:58 -0500 Received: by verein.lst.de (Postfix, from userid 2407) id 9E17968B05; Wed, 15 Jan 2020 09:38:54 +0100 (CET) Date: Wed, 15 Jan 2020 09:38:54 +0100 From: Christoph Hellwig To: David Howells Cc: linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, hch@lst.de, tytso@mit.edu, adilger.kernel@dilger.ca, darrick.wong@oracle.com, clm@fb.com, josef@toxicpanda.com, dsterba@suse.com, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: Problems with determining data presence by examining extents? Message-ID: <20200115083854.GB23039@lst.de> References: <4467.1579020509@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4467.1579020509@warthog.procyon.org.uk> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Tue, Jan 14, 2020 at 04:48:29PM +0000, David Howells wrote: > Again with regard to my rewrite of fscache and cachefiles: > > https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter > > I've got rid of my use of bmap()! Hooray! > > However, I'm informed that I can't trust the extent map of a backing file to > tell me accurately whether content exists in a file because: > > (a) Not-quite-contiguous extents may be joined by insertion of blocks of > zeros by the filesystem optimising itself. This would give me a false > positive when trying to detect the presence of data. > > (b) Blocks of zeros that I write into the file may get punched out by > filesystem optimisation since a read back would be expected to read zeros > there anyway, provided it's below the EOF. This would give me a false > negative. The whole idea of an out of band interface is going to be racy and suffer from implementation loss. I think what you want is something similar to the NFSv4.2 READ_PLUS operation - give me that if there is any and otherwise tell me that there is a hole. I think this could be a new RWF_NOHOLE or similar flag, just how to return the hole size would be a little awkward. Maybe return a specific negative error code (ENODATA?) and advance the iov anyway.