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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 E885AC04E87 for ; Mon, 20 May 2019 13:12:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B19F320815 for ; Mon, 20 May 2019 13:12:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="uDtd4HyR" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731135AbfETNMt (ORCPT ); Mon, 20 May 2019 09:12:49 -0400 Received: from mail-vs1-f66.google.com ([209.85.217.66]:38687 "EHLO mail-vs1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730513AbfETNMt (ORCPT ); Mon, 20 May 2019 09:12:49 -0400 Received: by mail-vs1-f66.google.com with SMTP id x184so7823909vsb.5; Mon, 20 May 2019 06:12:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=n/7WH964s/tfkaSzoQZwKHz45h3Le1MO2gu6rK8UR0U=; b=uDtd4HyR6SEm7XYY8zBiJinUfj7o/QJedsy2BfqAmP2wnCUyL/G9wyDuzwcCYWDV09 HRGRZxd+POBv54dWWw8vlUGlMUeNJ21HE2x8i0x62hFM4ccd4V16nJNu65QhuDdMaYRu lHaiS+avAsBPSGbkYEaB9BCFi87S2+wTD6YAeMIjKVvXgqqUYcI9xDsWcrSm1IFNfmgF TVChfocKTkSrK1gOO3uY0JhdiN6pFoCSy/zhi0ZSIyTyj3UuF3qUjuY/vvYmX78zWBOH 9XM+awSNT5LgghbDLkl4kDZsknB5WhIFNZXRT+6lz85IjRal3GrQcasjKL0pc6XNEtak X13A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=n/7WH964s/tfkaSzoQZwKHz45h3Le1MO2gu6rK8UR0U=; b=FatY43jjXG6hBzc2JdkV6PKDzEyll0pJ0swGSBz+kRPSsyu/NWHNcpT5GZHO+4Bm1r w6zNqfEAgMa4R7Du8DshHPMNYKrqAkRrUm6FSRjcAer6c6u3rCX7u5v91hhdXctsgL7a MfhVRSphHmHXDy94NTOUawYL41HY/MzSW/pE9XPHgPBnTPQ8oLn0rTzN66ZAUoprM0V5 L8xXTKlmyaK7NQC8fGfwn95u9rGt44Vm/88sMt1mb8+EydXYTyKLXUwLl7eIlIlYAJ7Q JEb2ZsolDx8kUMZChdmX39uKga4A9v3399ay7ZcmunZ0rc7e7dzjhPaa4i4sdKgAjj/+ zLNw== X-Gm-Message-State: APjAAAVccj1soOFhFjkDF/paP7AtMiyzwLCmZ5hqJScs69OWmKECjVeB NPMz+9IQhEhEWGKYWxUcD8AVHwtWZh/5Ds4G240= X-Google-Smtp-Source: APXvYqwS6zglHLFVLS7z0hunZ6Ksqz2jiiVdQhTdOoD41RAsjj8bwuHtdLmTK3A0H2CplJUpik/4SBBJxK/t7Vlzm4c= X-Received: by 2002:a05:6102:382:: with SMTP id m2mr15790274vsq.134.1558357967201; Mon, 20 May 2019 06:12:47 -0700 (PDT) MIME-Version: 1.0 References: <20181203083416.28978-1-david@fromorbit.com> <20181203083416.28978-2-david@fromorbit.com> <20181204151332.GA32245@infradead.org> <20181204212948.GO6311@dastard> <20181204223102.GR6311@dastard> In-Reply-To: From: Olga Kornievskaia Date: Mon, 20 May 2019 09:12:36 -0400 Message-ID: Subject: Re: [PATCH 01/11] vfs: copy_file_range source range over EOF should fail To: Amir Goldstein Cc: Dave Chinner , Christoph Hellwig , linux-fsdevel , linux-xfs , linux-nfs , overlayfs , ceph-devel@vger.kernel.org, CIFS Content-Type: text/plain; charset="UTF-8" Sender: linux-nfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org On Mon, May 20, 2019 at 5:10 AM Amir Goldstein wrote: > > On Wed, Dec 5, 2018 at 12:31 AM Dave Chinner wrote: > > > > On Tue, Dec 04, 2018 at 04:47:18PM -0500, Olga Kornievskaia wrote: > > > On Tue, Dec 4, 2018 at 4:35 PM Dave Chinner wrote: > > > > > > > > On Tue, Dec 04, 2018 at 07:13:32AM -0800, Christoph Hellwig wrote: > > > > > On Mon, Dec 03, 2018 at 02:46:20PM +0200, Amir Goldstein wrote: > > > > > > > From: Dave Chinner > > > > > > > > > > > > > > The man page says: > > > > > > > > > > > > > > EINVAL Requested range extends beyond the end of the source file > > > > > > > > > > > > > > But the current behaviour is that copy_file_range does a short > > > > > > > copy up to the source file EOF. Fix the kernel behaviour to match > > > > > > > the behaviour described in the man page. > > > > > > > > > > I think the behavior implemented is a lot more useful than the one > > > > > documented.. > > > > > > > > The current behaviour is really nasty. Because copy_file_range() can > > > > return short copies, the caller has to implement a loop to ensure > > > > the range hey want get copied. When the source range you are > > > > trying to copy overlaps source EOF, this loop: > > > > > > > > while (len > 0) { > > > > ret = copy_file_range(... len ...) > > > > ... > > > > off_in += ret; > > > > off_out += ret; > > > > len -= ret; > > > > } > > > > > > > > Currently the fallback code copies up to the end of the source file > > > > on the first copy and then fails the second copy with EINVAL because > > > > the source range is now completely beyond EOF. > > > > > > > > So, from an application perspective, did the copy succeed or did it > > > > fail? > > > > > > > > Existing tools that exercise copy_file_range (like xfs_io) consider > > > > this a failure, because the second copy_file_range() call returns > > > > EINVAL and not some "there is no more to copy" marker like read() > > > > returning 0 bytes when attempting to read beyond EOF. > > > > > > > > IOWs, we cannot tell the difference between a real error and a short > > > > copy because the input range spans EOF and it was silently > > > > shortened. That's the API problem we need to fix here - the existing > > > > behaviour is really crappy for applications. Erroring out > > > > immmediately is one solution, and it's what the man page says should > > > > happen so that is what I implemented. > > > > > > > > Realistically, though, I think an attempt to read beyond EOF for the > > > > copy should result in behaviour like read() (i.e. return 0 bytes), > > > > not EINVAL. The existing behaviour needs to change, though. > > > > > > There are two checks to consider > > > 1. pos_in >= EOF should return EINVAL > > > 2. however what's perhaps should be relaxed is pos_in+len >= EOF > > > should return a short copy. > > > > > > Having check#1 enforced allows to us to differentiate between a real > > > error and a short copy. > > > > That's what the code does right now and *exactly what I'm trying to > > fix* because it EINVAL is ambiguous and not an indicator that we've > > reached the end of the source file. EINVAL can indicate several > > different errors, so it really has to be treated as a "copy failed" > > error by applications. > > > > Have a look at read/pread() - they return 0 in this case to indicate > > a short read, and the value of zero is explicitly defined as meaning > > "read position is beyond EOF". Applications know straight away that > > there is no more data to be read and there was no error, so can > > terminate on a successful short read. > > > > We need to allow applications to terminate copy loops on a > > successful short copy. IOWs, applications need to either: > > > > - get an immediate error saying the range is invalid rather > > than doing a short copy (as per the man page); or > > - have an explicit marker to say "no more data to be copied" > > > > Applications need the "no more data to copy" case to be explicit and > > unambiguous so they can make sane decisions about whether a short > > copy was successful because the file was shorter than expected or > > whether a short copy was a result of a real error being encountered. > > The current behaviour is largely unusable for applications because > > they have to guess at the reason for EINVAL part way through a > > copy.... > > > > Dave, > > I went a head and implemented the desired behavior. > However, while testing I observed that the desired behavior is already > the existing behavior. For example, trying to copy 10 bytes from a 2 bytes file, > xfs_io copy loop ends as expected: > copy_file_range(4, [0], 3, [0], 10, 0) = 2 > copy_file_range(4, [2], 3, [2], 8, 0) = 0 > > This was tested on ext4 and xfs with reflink on recent kernel as well as on > v4.20-rc1 (era of original patch set). > > Where and how did you observe the EINVAL behavior described above? > (besides man page that is). There are even xfstests (which you modified) > that verify the return 0 for past EOF behavior. > > For now, I am just dropping this patch from the patch series. > Let me know if I am missing something. The was fixing inconsistency in what the man page specified (ie., it must fail with EINVAL if offsets are out of range) which was never enforced by the code. The patch then could be to fix the existing semantics (man page) of the system call. Copy file range range is not only read and write but rather lseek+read+write and if somebody specifies an incorrect offset to the lseek the system call should fail. Thus I still think that copy file range should enforce that specifying a source offset beyond the end of the file should fail with EINVAL. If the copy file range returned 0 bytes does it mean it's a stopping condition, not according to the current semantics.