From nobody Sun Feb 8 11:45:36 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 B5AA6EB64DD for ; Wed, 5 Jul 2023 06:37:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231577AbjGEGhW (ORCPT ); Wed, 5 Jul 2023 02:37:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47050 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230374AbjGEGhU (ORCPT ); Wed, 5 Jul 2023 02:37:20 -0400 Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DB7821700 for ; Tue, 4 Jul 2023 23:37:18 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id 3f1490d57ef6-c4e5e8093a2so3689396276.2 for ; Tue, 04 Jul 2023 23:37:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1688539038; x=1691131038; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:from:to:cc:subject:date:message-id :reply-to; bh=kEcsCBzNF4o70240RbNtRsxkJMv3dgoeG47F/eJErR4=; b=NRt67G4eRCmPFMXhxD3qW9K9EDj0xnVdQKTKufRvQkcPo7ZdLBv0zTj1I+0oIpa2hf RNbwbPBRRogvAWVpk5UaiqbfxSKUz15RoJ7CB+USDdwHmnmfpV6jkLxbEnsYpFFZaO/V SHkrXnyB7eFzXg59ZHInsE6Ag/21l5iBDRtfRUxDxPZ/FU1OKcIdaR+keSSWazvkJh5Q 7s+3RLuRM38qG/NcRSerlS841aoCJMp0+XIoBRVOSxggq4iZYaH+zqxDmhUtoJFuHlZu D53jP1OcRKF4mM/4S+ltgmuuTXWPdfIQ3Qrpc4LEr92kprtgKQxmqXBhxsp8tdb8AsgV 55rw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1688539038; x=1691131038; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=kEcsCBzNF4o70240RbNtRsxkJMv3dgoeG47F/eJErR4=; b=VnjvvCmzf69SQTBgCZs3RISXrhsnJlJgzrvbC0tDwpSCI0SyB1hjIpTIRpSYSIv24s oAKw4exigoVoVfGAqEgBRtFkLX1+PgrUGX5nrleqFZrNIFLLHF+Zdt4vR2IruWF2Lnbx nitekjTFhlPWjkOEK3pyjs7UQlUtLWaEdX9XFa7CE31uBPQtc5b4gyhAQQou1D6eFmbb 1b/ro5aVGYERVIjS0m4eaiE9t2Z5NzV+fdDrk5ZjQ0MqjPnuPIYgzLQSx1MyNd/KoOIM A0ddH9OJeY5JnGzbjfrvhE3DC+xlTYKjgobhE2T08JOzZ43IsAmpOHKIWN3rD05j3qke fESw== X-Gm-Message-State: ABy/qLbyQocTr7WhuXjtCVr9lH6n3QxDDpkVPVCly4yycqePlf+r/6cw le7LfVbXmnJeubJJUSkpcOw7o/183+w= X-Google-Smtp-Source: APBJJlFxf2AFzH5E1himxE/0ay/K/ncJrIflv9U1K8XKuqoxYpjdV/ZXI10Rc5HBkhf6oCEZCsT7XVeCpvM= X-Received: from surenb-desktop.mtv.corp.google.com ([2620:15c:211:201:9164:ef9f:8918:e2b6]) (user=surenb job=sendgmr) by 2002:a25:ad96:0:b0:c5d:5b6f:f5c5 with SMTP id z22-20020a25ad96000000b00c5d5b6ff5c5mr31841ybi.4.1688539038122; Tue, 04 Jul 2023 23:37:18 -0700 (PDT) Date: Tue, 4 Jul 2023 23:37:10 -0700 In-Reply-To: <20230705063711.2670599-1-surenb@google.com> Mime-Version: 1.0 References: <20230705063711.2670599-1-surenb@google.com> X-Mailer: git-send-email 2.41.0.255.g8b1d071c50-goog Message-ID: <20230705063711.2670599-2-surenb@google.com> Subject: [PATCH v2 1/2] fork: lock VMAs of the parent process when forking From: Suren Baghdasaryan To: akpm@linux-foundation.org Cc: jirislaby@kernel.org, jacobly.alt@gmail.com, holger@applied-asynchrony.com, hdegoede@redhat.com, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, chriscli@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, rppt@kernel.org, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Suren Baghdasaryan Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" When forking a child process, parent write-protects an anonymous page and COW-shares it with the child being forked using copy_present_pte(). Parent's TLB is flushed right before we drop the parent's mmap_lock in dup_mmap(). If we get a write-fault before that TLB flush in the parent, and we end up replacing that anonymous page in the parent process in do_wp_page() (because, COW-shared with the child), this might lead to some stale writable TLB entries targeting the wrong (old) page. Similar issue happened in the past with userfaultfd (see flush_tlb_page() call inside do_wp_page()). Lock VMAs of the parent process when forking a child, which prevents concurrent page faults during fork operation and avoids this issue. This fix can potentially regress some fork-heavy workloads. Kernel build time did not show noticeable regression on a 56-core machine while a stress test mapping 10000 VMAs and forking 5000 times in a tight loop shows ~5% regression. If such fork time regression is unacceptable, disabling CONFIG_PER_VMA_LOCK should restore its performance. Further optimizations are possible if this regression proves to be problematic. Suggested-by: David Hildenbrand Reported-by: Jiri Slaby Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@ke= rnel.org/ Reported-by: Holger Hoffst=C3=A4tte Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@ap= plied-asynchrony.com/ Reported-by: Jacob Young Closes: https://bugzilla.kernel.org/show_bug.cgi?id=3D217624 Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first") Cc: stable@vger.kernel.org Signed-off-by: Suren Baghdasaryan Acked-by: David Hildenbrand --- kernel/fork.c | 1 + 1 file changed, 1 insertion(+) diff --git a/kernel/fork.c b/kernel/fork.c index b85814e614a5..d2e12b6d2b18 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -686,6 +686,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *= mm, for_each_vma(old_vmi, mpnt) { struct file *file; =20 + vma_start_write(mpnt); if (mpnt->vm_flags & VM_DONTCOPY) { vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt)); continue; --=20 2.41.0.255.g8b1d071c50-goog