From nobody Mon Feb 9 10:28:40 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 242B9C7EE23 for ; Wed, 31 May 2023 22:08:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230423AbjEaWIn (ORCPT ); Wed, 31 May 2023 18:08:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60146 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230323AbjEaWIh (ORCPT ); Wed, 31 May 2023 18:08:37 -0400 Received: from mail-qv1-xf2f.google.com (mail-qv1-xf2f.google.com [IPv6:2607:f8b0:4864:20::f2f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 628BB139 for ; Wed, 31 May 2023 15:08:19 -0700 (PDT) Received: by mail-qv1-xf2f.google.com with SMTP id 6a1803df08f44-6260bb94363so1687256d6.0 for ; Wed, 31 May 2023 15:08:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; t=1685570898; x=1688162898; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=xcpMDk08TnAKM11DZYaTQHoJHdMHYlx0PmCfJSmuqxQ=; b=wEwknpWuACfVk2BLZAAwhK1P+A8Agyg2LkaiMrZv/vk+Jqv3PjEA3zHf+KOHmK4Pxx sYYbfV/UmdJ7qbsavZh91UeTFvV/aLbtLtNOxKYXZP0bQ+FrDbx/ezrzjN+6DuOM96+6 CyyRczA9px0WqR6nr3eZ09ZYqir15Ii3x7h8o= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1685570898; x=1688162898; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=xcpMDk08TnAKM11DZYaTQHoJHdMHYlx0PmCfJSmuqxQ=; b=dxRU9l108q8FcG0R4PU8QybbwoRVEfjf/ynTnu5cJ6z5nQf+GRTBO8K5LBOBk41SKj 7ftxc7vub07Iazd/SX5n8Ip/U8DQcxpmNaUpJNskbfJETcEiPKdI0LqBY5BadfX0JKpj HmkJrj/RW6IWsu1a1w7VhSxQO0oVv0i5UVP8+NY88NxQMUP4xBeoQ3ezf3sykR+Nk4i0 hmQkCoz/JRNaqL4R8xvIhMm/vFybpHIcm9lzoDLheNbLHgS6iA5gXhc2osHhj7D73ZQo nSL4y8yIbdAQhyH3hQ2LfNlLfuSKWADwd5lldVtZZ8lcZcbwU77DFApV3Y4kbRNBF+Rj CofQ== X-Gm-Message-State: AC+VfDz1zS0LoMVNUNXOz7Xh3c8Z4n2pEbuGEaXpPL8TgFEkunipmdYH g4zDE5xtM7PkRKxJl09xBIDASZnZas9luMvJ604= X-Google-Smtp-Source: ACHHUZ6VD/g4jSj2d9B84SOdyc6mJVRalNKUWWpk2+9CxmTsHgUmiKwb5GcyFfj7v6tYE/kvvmPfuQ== X-Received: by 2002:a05:6214:509d:b0:570:5389:2216 with SMTP id kk29-20020a056214509d00b0057053892216mr16116075qvb.22.1685570898020; Wed, 31 May 2023 15:08:18 -0700 (PDT) Received: from joelboxx.c.googlers.com.com (129.239.188.35.bc.googleusercontent.com. [35.188.239.129]) by smtp.gmail.com with ESMTPSA id f2-20020ac87f02000000b003f6b0f4126fsm6666172qtk.8.2023.05.31.15.08.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 31 May 2023 15:08:17 -0700 (PDT) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org Cc: "Joel Fernandes (Google)" , Linus Torvalds , linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Shuah Khan , Vlastimil Babka , Michal Hocko , Lorenzo Stoakes , Kirill A Shutemov , "Liam R. Howlett" , "Paul E. McKenney" , Suren Baghdasaryan , Kalesh Singh , Lokesh Gidra , Vineeth Pillai Subject: [PATCH v4 1/7] mm/mremap: Optimize the start addresses in move_page_tables() Date: Wed, 31 May 2023 22:08:01 +0000 Message-ID: <20230531220807.2048037-2-joel@joelfernandes.org> X-Mailer: git-send-email 2.41.0.rc2.161.g9c6817b8e7-goog In-Reply-To: <20230531220807.2048037-1-joel@joelfernandes.org> References: <20230531220807.2048037-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Recently, we see reports [1] of a warning that triggers due to move_page_tables() doing a downward and overlapping move on a mutually-aligned offset within a PMD. By mutual alignment, I mean the source and destination addresses of the mremap are at the same offset within a PMD. This mutual alignment along with the fact that the move is downward is sufficient to cause a warning related to having an allocated PMD that does not have PTEs in it. This warning will only trigger when there is mutual alignment in the move operation. A solution, as suggested by Linus Torvalds [2], is to initiate the copy process at the PMD level whenever such alignment is present. Implementing this approach will not only prevent the warning from being triggered, but it will also optimize the operation as this method should enhance the speed of the copy process whenever there's a possibility to start copying at the PMD level. Some more points: a. The optimization can be done only when both the source and destination of the mremap do not have anything mapped below it up to a PMD boundary. I add support to detect that. b. #a is not a problem for the call to move_page_tables() from exec.c as nothing is expected to be mapped below the source. However, for non-overlapping mutually aligned moves as triggered by mremap(2), I added support for checking such cases. c. I currently only optimize for PMD moves, in the future I/we can build on this work and do PUD moves as well if there is a need for this. But I want to take it one step at a time. d. We need to be careful about mremap of ranges within the VMA itself. For this purpose, I added checks to determine if the address to align is not the beginning of the VMA which that address corresponds to. [1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/ [2] https://lore.kernel.org/all/CAHk-=3Dwhd7msp8reJPfeGNyt0LiySMT0egExx3TVZ= SX3Ok6X=3D9g@mail.gmail.com/ Suggested-by: Linus Torvalds Signed-off-by: Joel Fernandes (Google) --- mm/mremap.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 61 insertions(+) diff --git a/mm/mremap.c b/mm/mremap.c index 411a85682b58..bf355e4d6bd4 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -478,6 +478,51 @@ static bool move_pgt_entry(enum pgt_entry entry, struc= t vm_area_struct *vma, return moved; } =20 +/* + * A helper to check if a previous mapping exists. Required for + * move_page_tables() and realign_addr() to determine if a previous mapping + * exists before we can do realignment optimizations. + */ +static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_= to_align, + unsigned long mask) +{ + unsigned long addr_masked =3D addr_to_align & mask; + struct vm_area_struct *prev =3D NULL, *cur =3D NULL; + + /* + * If @addr_to_align of either source or destination is not the beginning + * of the corresponding VMA, we can't align down or we will destroy part + * of the current mapping. + */ + if (vma->vm_start !=3D addr_to_align) + return false; + + /* + * Find the VMA before @vma to see if it subsumes the masked address. + * The mmap write lock is held here so the lookup is safe. + */ + cur =3D find_vma_prev(vma->vm_mm, vma->vm_start, &prev); + if (WARN_ON_ONCE(cur !=3D vma)) + return false; + + return !prev || prev->vm_end <=3D addr_masked; +} + +/* Opportunistically realign to specified boundary for faster copy. */ +static void realign_addr(unsigned long *old_addr, struct vm_area_struct *o= ld_vma, + unsigned long *new_addr, struct vm_area_struct *new_vma, + unsigned long mask) +{ + bool mutually_aligned =3D (*old_addr & ~mask) =3D=3D (*new_addr & ~mask); + + if ((*old_addr & ~mask) && mutually_aligned + && can_align_down(old_vma, *old_addr, mask) + && can_align_down(new_vma, *new_addr, mask)) { + *old_addr =3D *old_addr & mask; + *new_addr =3D *new_addr & mask; + } +} + unsigned long move_page_tables(struct vm_area_struct *vma, unsigned long old_addr, struct vm_area_struct *new_vma, unsigned long new_addr, unsigned long len, @@ -493,6 +538,15 @@ unsigned long move_page_tables(struct vm_area_struct *= vma, =20 old_end =3D old_addr + len; =20 + /* + * If possible, realign addresses to PMD boundary for faster copy. + * Don't align for intra-VMA moves as we may destroy existing mappings. + */ + if ((vma !=3D new_vma) + && (len >=3D PMD_SIZE - (old_addr & ~PMD_MASK))) { + realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK); + } + if (is_vm_hugetlb_page(vma)) return move_hugetlb_page_tables(vma, new_vma, old_addr, new_addr, len); @@ -565,6 +619,13 @@ unsigned long move_page_tables(struct vm_area_struct *= vma, =20 mmu_notifier_invalidate_range_end(&range); =20 + /* + * Prevent negative return values when {old,new}_addr was realigned + * but we broke out of the above loop for the first PMD itself. + */ + if (len + old_addr < old_end) + return 0; + return len + old_addr - old_end; /* how much done */ } =20 --=20 2.41.0.rc2.161.g9c6817b8e7-goog