From nobody Fri Jun 19 23:43:33 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 03EFAC433F5 for ; Fri, 25 Mar 2022 21:58:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233658AbiCYWAU (ORCPT ); Fri, 25 Mar 2022 18:00:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35630 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233635AbiCYWAS (ORCPT ); Fri, 25 Mar 2022 18:00:18 -0400 Received: from ms.lwn.net (ms.lwn.net [45.79.88.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9404C118F5A; Fri, 25 Mar 2022 14:58:43 -0700 (PDT) Received: from localhost (unknown [IPv6:2601:281:8300:35::5f6]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ms.lwn.net (Postfix) with ESMTPSA id 454BB2C3; Fri, 25 Mar 2022 21:58:43 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 ms.lwn.net 454BB2C3 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=lwn.net; s=20201203; t=1648245523; bh=hsYCabDYG9BsnSr6aIERiuj385zksjChMI7/4113Z3I=; h=From:To:Cc:Subject:Date:From; b=VrC4nDK2COPGZVaDKnzQ0RIroS8+LdFo30Izn8Ow21Zvvjs6AqQjlDJVidAdf4GsO SVp8q2DV2vdvwXDMXrYcOnCR+7JQcqksOye/BVL/VSsQ4ExGakHB5wDu7TI59QEntr 8Y0SN1IqWmveU5eCyRyg6JuBCyt0Ky9DQigJT0p1yfgjemOTeEdkMshz5qptfIODsq Ba6yF7SNSH6EF5vft5y7yrnb5nDZDxAZ0keNFYmMyCfbsdVdSLRTKjfPxC+88NwKyG Ljfm/tbMjb6OVuKq82e3Vm8m0gqjM95BaDh6rH+nDwdd5/4Ys1Ecka/FtELk6jcKQ8 jnqG8bh59RWMQ== From: Jonathan Corbet To: linux-doc@vger.kernel.org Cc: Linus Torvalds , linux-kernel@vger.kernel.org Subject: [PATCH] docs: Add a document on how to fix a messy diffstat Date: Fri, 25 Mar 2022 15:58:42 -0600 Message-ID: <87wnghd78t.fsf@meer.lwn.net> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" A branch with merges in will sometimes create a diffstat containing a lot of unrelated work at "git request-pull" time. Create a document based on Linus's advice (found in the links below) and add it to the maintainer manual in the hope of saving some wear on Linus's keyboard going forward. Link: https://lore.kernel.org/lkml/CAHk-=3Dwg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_J= f7ViH33Sw7PHA@mail.gmail.com/ Link: https://lore.kernel.org/lkml/CAHk-=3DwgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQ= WRBN4_CUPr=3DeQ@mail.gmail.com/ Signed-off-by: Jonathan Corbet --- [If this passes muster I'll likely toss a version onto LWN as well] Documentation/maintainer/index.rst | 1 + Documentation/maintainer/messy-diffstat.rst | 96 +++++++++++++++++++++ 2 files changed, 97 insertions(+) create mode 100644 Documentation/maintainer/messy-diffstat.rst diff --git a/Documentation/maintainer/index.rst b/Documentation/maintainer/= index.rst index f0a60435b124..3e03283c144e 100644 --- a/Documentation/maintainer/index.rst +++ b/Documentation/maintainer/index.rst @@ -12,6 +12,7 @@ additions to this manual. configure-git rebasing-and-merging pull-requests + messy-diffstat maintainer-entry-profile modifying-patches =20 diff --git a/Documentation/maintainer/messy-diffstat.rst b/Documentation/ma= intainer/messy-diffstat.rst new file mode 100644 index 000000000000..970eac087f67 --- /dev/null +++ b/Documentation/maintainer/messy-diffstat.rst @@ -0,0 +1,96 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Handling messy pull-request diffstats +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Subsystem maintainers routinely use ``git request-pull`` as part of the +process of sending work upstream. Normally, the result includes a nice +diffstat listing showing which files will be touched and how much of each +will be changed. Occasionally, though, a repository with a relatively +complicated development history will yield a massive diffstat containing a +great deal of unrelated work. The result looks ugly and obscures what the +pull request is actually doing. This document describes what is happening +and how to fix things up; it is derived from The Wisdom of Linus Torvalds, +found in Linus1_ and Linus2_. + +.. _Linus1: https://lore.kernel.org/lkml/CAHk-=3Dwg3wXH2JNxkQi+eLZkpuxqV+w= PiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/ +.. _Linus2: https://lore.kernel.org/lkml/CAHk-=3DwgXbSa8yq8Dht8at+gxb_idnJ= 7X5qWZQWRBN4_CUPr=3DeQ@mail.gmail.com/ + +A Git development history proceeds as a series of commits. In a simplified +manner, mainline kernel development looks like this:: + + ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN + +If one wants to see what has changed between two points, a command like +will do the job:: + + $ git diff --stat --summary vN-rc2..vN-rc3 + +Here, there are two clear points in the history; Git will essentially +"subtract" the beginning point from the end point and display the resulting +differences. The requested operation is unambiguous and easy enough to +understand. + +When a subsystem maintainer creates a branch and commits changes to it, the +result in the simplest case is a history that looks like:: + + ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN + | + +-- c1 --- c2 --- ... --- cN + +If that maintainer now uses ``git diff`` to see what has changed between +the mainline branch (let's call it "linus") and cN, there are still two +clear endpoints, and the result is as expected. So a pull request +generated with ``git request-pull`` will also be as expected. But now +consider a slightly more complex development history:: + + ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN + | | + | +-- c1 --- c2 --- ... --- cN + | / + +-- x1 --- x2 --- x3 + +Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the +two were then subsequently merged into c2. Now a pull request generated +for cN may end up being messy indeed, and developers often end up wondering +why. + +What is happening here is that there are no longer two clear end points for +the ``git diff`` operation to use. The development culminating in cN +started in two different places; to generate the diffstat, ``git diff`` +ends up having pick one of them and hoping for the best. If the diffstat +starts at vN-rc1, it may end up including all of the changes between there +and the second origin end point (vN-rc2), which is certainly not what our +maintainer had in mind. With all of that extra junk in the diffstat, it +may be impossible to tell what actually happened in the changes leading up +to cN. + +Maintainers often try to resolve this problem by, for example, rebasing the +branch or performing another merge with the linus branch, then recreating +the pull request. This approach tends not to lead to joy at the receiving +end of that pull request; rebasing and/or merging just before pushing +upstream is a well-known way to get a grumpy response. + +So what is to be done? The best response when confronted with this +situation is to indeed to a merge, but to do it privately, as if it were +the source of shame. Create a new, throwaway branch and do the merge +there:: + + ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN + | | | + | +-- c1 --- c2 --- ... --- cN | + | / | | + +-- x1 --- x2 --- x3 +------------+-- TEMP + +The merge operation resolves all of the complications resulting from the +multiple beginning points, yielding a coherent result that contains only +the differences from the mainline branch. Now it will be possible to +generate a diffstat with the desired information:: + + $ git diff -C --stat --summary linus..TEMP + +Save the output from this command, then simply delete the TEMP branch; +definitely do not expose it to the outside world. Take the saved diffstat +output and edit it into the messy pull request, yielding a result that +shows what is really going on. That request can then be sent upstream. --=20 2.35.1