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=-2.8 required=3.0 tests=BAYES_00,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=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 4549CC4338F for ; Tue, 17 Aug 2021 12:19:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1FF5460F38 for ; Tue, 17 Aug 2021 12:19:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237014AbhHQMUK (ORCPT ); Tue, 17 Aug 2021 08:20:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54732 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234866AbhHQMUH (ORCPT ); Tue, 17 Aug 2021 08:20:07 -0400 Received: from mail-wm1-x32a.google.com (mail-wm1-x32a.google.com [IPv6:2a00:1450:4864:20::32a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 75CE8C061764 for ; Tue, 17 Aug 2021 05:19:32 -0700 (PDT) Received: by mail-wm1-x32a.google.com with SMTP id w24so5804341wmi.5 for ; Tue, 17 Aug 2021 05:19:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:date:to:subject:message-id:mime-version:content-disposition; bh=6ENXzKZr3llo+VrC9R9YSqAe2/24cY2AWp8C2U3ZFbo=; b=HDPzkcxl6uaS0otfcmjZWZMP9jQzSIg3fmqu0WU/HjDj5KCMLovUuJESRE9gkca9Pj Wkn5wASBPQoxigV6+D/9tuJnQtjd8xQqN2jA7dRcIYk35a8Kg3MCKXuMJ4nsihNr5ThQ d3CGoXtV7JGcLXNflDTZDtct4nNJ6YPOwXPlLsRuXmjtKMAWVd6+jryhAThttEvcK26O r7mSe9mqMESb3tHG3dWcYIzdgYVH+/UcuxUekCyTv17TtJUccnwOQcnwlefZijgrA2Pu 5673Kz2OvgW0HwXs2USXtSc7dMAU5TNplM7Vow/ce4yedvcOuFHa5Di9nmNd/2lUo3nc zVaQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:date:to:subject:message-id:mime-version :content-disposition; bh=6ENXzKZr3llo+VrC9R9YSqAe2/24cY2AWp8C2U3ZFbo=; b=EHzAVYpH/8aRwP/WJFl9y1g+UObDyTOwXSUZjXzYBn3ZyoNaIvEdGzo7bkAFyoqPOc kHdk0JYOXNJhR9+gOxhGObYiHk59OWkse1QkBd/9c5BU8Mrz9P5AGLmu0Kq/dwXV/Fia ceI00bVXN0dBw9EWq4mKOlVSt3LSu/qhG4bQWIFqV8pnV5NgkpO4amKBTjxBnroesvsG FDUvFWP5jg8N6Lk0JACTylx35KVzUovYf2VF7gO33H2EcaL9E1dFiA+SNteeAlQ78TuE wazmu3OJGmsPn5xhAM//Tm8GJXiK0Hw0BWHvFX6tmz4IZMjTmo+D1MU9LZZ2cNBuE8o1 W95Q== X-Gm-Message-State: AOAM530FghNjTQpM8Ur9D6MV6jwB00sutquwNXkGeX1yXiXernNh+EGa +fbuXEP6vkHcyBQi066bkyNlP/vPsmE= X-Google-Smtp-Source: ABdhPJzkf7vLvI71vtHvUG5G73rEk0lTN3GM5kylPRzy1kzvmkv6nUbyt7vIaEAAW745LNVCf+F80A== X-Received: by 2002:a1c:1f07:: with SMTP id f7mr3020389wmf.116.1629202771120; Tue, 17 Aug 2021 05:19:31 -0700 (PDT) Received: from saturne.home ([2a01:cb1d:16b:7e00:254e:383d:631e:66b2]) by smtp.gmail.com with ESMTPSA id l12sm1800304wms.24.2021.08.17.05.19.29 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Aug 2021 05:19:29 -0700 (PDT) From: Laurent Stacul X-Google-Original-From: Laurent Stacul Date: Tue, 17 Aug 2021 14:19:12 +0200 To: linux-kernel@vger.kernel.org Subject: XFS/mmap reflink file question Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, I spent much time digging into the mmap mechanism and I don't have a clear view on mmap'ing a file and a reflink to this file would be mapped twice in memory (this only applies in case the filesystem supports reflink feature like XFS). To describe my tests, I generate a file stored on an XFS partition and create a reflink of it: % dd if=/dev/zero of=./output.dat bs=1M count=24 % cp --reflink -v output.dat output2.dat % xfs_bmap -v output.dat output.dat: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL 0: [0..49151]: 3756776..3805927 0 (3756776..3805927) 49152 100000 % xfs_bmap -v output2.dat output2.dat: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL 0: [0..49151]: 3756776..3805927 0 (3756776..3805927) 49152 100000 Then I mmap the first file twice using vmtouch tool: % vmtouch -l output.dat& [1] 15870 LOCKED 6144 pages (24M) % vmtouch -l output.dat& [2] 15872 LOCKED 6144 pages (24M) % pmap -X 15872 | grep -e 'Pss' -e 'output' | awk '{if(NR>1)printf("%16s %4s %6s %10s %10s %10s\n", $1, $2, $4, $5, $7, $8)}' Address Perm Device Inode Rss Pss 7fcbb9eb9000 r--s fc:10 3755268 24576 12288 As we can see the Proportional Set Size is as expected the half of the Resident Set Size because the memory is shared by the two processes. Now, I mmap the reflink `output2.dat' of 'output.dat': % vmtouch -l output2.dat& [3] 15892 LOCKED 6144 pages (24M) % pmap -X 15872 | grep -e 'Pss' -e 'output' | awk '{if(NR>1)printf("%16s %4s %6s %10s %10s %10s\n", $1, $2, $4, $5, $7, $8)}' Address Perm Device Inode Rss Pss 7fcbb9eb9000 r--s fc:10 3755268 24576 12288 The Pss of mmap'ed file by the first process has not decreased (I expected a value of Rss / 3 because I hoped the memory would have been shared by the 3 processes). If I look at the process map of the last process, we can interpret a new memory area was allocated and locked. % pmap -X 15892 | grep -e 'Pss' -e 'output' | awk '{if(NR>1)printf("%16s %4s %6s %10s %10s %10s\n", $1, $2, $4, $5, $7, $8)}' Address Perm Device Inode Rss Pss 7f5adc53f000 r--s fc:10 3755269 24576 24576 So my questions: - Why can't we benefit from the memory sharing when reflinked files are mmap'ed ? It would be great because one application would be, in the context of containers, the possibility to share some read only areas between container that are built from the layer diff that are reproducible between images. We can imagine a layer that brings some shared libraries in an image from a reproducible FS diff so that containers would not load several times a library. - I can think of many tricky cases with the behavior I was expecting (especially if a process has write access to the mapped area), but if you know a way, an option something to achieve what I am trying to do, I would be glad to hear it. - Conversely, don't hesitate to tell me my expectation is just crazy. Anyway, I am always looking forward to listening to valuable specialist insights. Thanks in advance, stac PS: Please, add me is CC if this message deserves an answer.