From nobody Sun Feb 8 12:37:00 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 8DB4FC0015E for ; Wed, 5 Jul 2023 17:12:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231902AbjGERMY (ORCPT ); Wed, 5 Jul 2023 13:12:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38728 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231685AbjGERMV (ORCPT ); Wed, 5 Jul 2023 13:12:21 -0400 Received: from mail-yw1-x114a.google.com (mail-yw1-x114a.google.com [IPv6:2607:f8b0:4864:20::114a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1BC381981 for ; Wed, 5 Jul 2023 10:12:20 -0700 (PDT) Received: by mail-yw1-x114a.google.com with SMTP id 00721157ae682-5706641dda9so67872937b3.3 for ; Wed, 05 Jul 2023 10:12:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1688577139; x=1691169139; 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=LEfYN9pWKwKFx1JeQhdsclXOpUTR5qIj3GofsUdOz+Q=; b=pv5J0UpQIPe0CfYpd/A4LUOIsPSqjggyBSXP8KA0L3IjO09ETnpFoKvsG9qpEBU9te LkkAk34KlR77CbwUqvjVmP+PelArfdSViArec9GtraLonPbdZCHYpxl59mhuJh+qwNH/ x8PVMMrUZyzXU+40IUVTClHb66yk1cJcLC9UceptakgAwptMXgWNfem+82VsI8PV2343 ulU6/+r+EU8f0/mTbgobEHK1XRAYgNrYZAN4gmcUtuXNe9XdnDjCwCv5EvDBO523pj4K 7SMAC2TVEkWPpnJYpSek3AANRder6w/kKNU3IvQKMe889P3MVrbJ0gLpEvPDPOLNjhs0 dzJA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1688577139; x=1691169139; 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=LEfYN9pWKwKFx1JeQhdsclXOpUTR5qIj3GofsUdOz+Q=; b=FCWKglAveXzTCASsx4JOanL7f+dU8kIfHq8eApJMaXyJWhJ4XSfX4WUxQOAv+ybJ8O WlsToJ/mw9KTCCncOgnD1JtBlrmNC/+JeZguWw4RHw0ElG943W+0bJY8JMe+aTzrm8pw u33ntMDA/JtB7E7Qyo2IVCocatlN/6AOARbJjjPog8dBXaJwR2zTN1RviNwwplUEw47I i3wkEBjjoE6x+k2xBoH+7WAuIXeW4ieRRC900D3qbfGalOrAagzHr66Aee4uhK9OCW8B 5VFEa5wXEF0v8n1EgXovqXzLuuGuwcCkqBUxH44PF3uolsWKs5bZU4A+BKcFbB57kqE7 0siw== X-Gm-Message-State: ABy/qLYaBkF70TzAVr7jnhdc7Ugd/gaQr2BkNMXn/kqrAca5VRNEvQuO gNPj6yPVSInQT2hviAedCEB3Lr1Wf90= X-Google-Smtp-Source: APBJJlE4UJqW+lKx2tf9OzDDJfg9cAcjpV6MDLC4+UeP6EFuD+QsQM5ZG6pzBAqVEKNmfN+tN2GiB5xNqjQ= X-Received: from surenb-desktop.mtv.corp.google.com ([2620:15c:211:201:946c:be30:90d9:9093]) (user=surenb job=sendgmr) by 2002:a81:b60f:0:b0:577:617b:f881 with SMTP id u15-20020a81b60f000000b00577617bf881mr98615ywh.8.1688577139225; Wed, 05 Jul 2023 10:12:19 -0700 (PDT) Date: Wed, 5 Jul 2023 10:12:11 -0700 In-Reply-To: <20230705171213.2843068-1-surenb@google.com> Mime-Version: 1.0 References: <20230705171213.2843068-1-surenb@google.com> X-Mailer: git-send-email 2.41.0.255.g8b1d071c50-goog Message-ID: <20230705171213.2843068-2-surenb@google.com> Subject: [PATCH v3 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 | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/fork.c b/kernel/fork.c index b85814e614a5..403bc2b72301 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(struct mm_struct = *mm, retval =3D -EINTR; goto fail_uprobe_end; } +#ifdef CONFIG_PER_VMA_LOCK + /* Disallow any page faults before calling flush_cache_dup_mm */ + for_each_vma(old_vmi, mpnt) + vma_start_write(mpnt); + vma_iter_init(&old_vmi, oldmm, 0); +#endif flush_cache_dup_mm(oldmm); uprobe_dup_mmap(oldmm, mm); /* --=20 2.41.0.255.g8b1d071c50-goog