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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, URIBL_RED autolearn=unavailable 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 B925DC433C1 for ; Thu, 25 Mar 2021 04:37:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9177461A1A for ; Thu, 25 Mar 2021 04:37:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229709AbhCYEhL (ORCPT ); Thu, 25 Mar 2021 00:37:11 -0400 Received: from mail.kernel.org ([198.145.29.99]:43886 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229716AbhCYEgo (ORCPT ); Thu, 25 Mar 2021 00:36:44 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id D2D1561A1A; Thu, 25 Mar 2021 04:36:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1616647004; bh=PUcF3KgHrzGiBtVp9wFO0dBtzI7xyYmREqwbxHQEUvQ=; h=Date:From:To:Subject:In-Reply-To:From; b=GwM9R8uzoniZV/WgrVnx1R8ejtH3mS1fOv/MBfU7+FDQIqydhUFxcSG5X0yZO6fbx QMrDFerayDX9coLgDwTcOr4p7awuTvNYVVdVV5ZyOkIIY6h/k9o29d70Li8KX7DjEG OJ2bYblYjr49i0Wz9UsnR2nh6TRMtGXM9PsFbJzI= Date: Wed, 24 Mar 2021 21:36:43 -0700 From: Andrew Morton To: aarcange@redhat.com, akpm@linux-foundation.org, bgardon@google.com, dimitri.sivanich@hpe.com, hannes@cmpxchg.org, jgg@nvidia.com, jgg@ziepe.ca, jglisse@redhat.com, linux-mm@kvack.org, mhocko@suse.com, mm-commits@vger.kernel.org, rientjes@google.com, seanjc@google.com, torvalds@linux-foundation.org Subject: [patch 03/14] mm/mmu_notifiers: ensure range_end() is paired with range_start() Message-ID: <20210325043643.dkjDXH1Bc%akpm@linux-foundation.org> In-Reply-To: <20210324213324.fe1ba237e471b5de17a08775@linux-foundation.org> User-Agent: s-nail v14.8.16 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org =46rom: Sean Christopherson Subject: mm/mmu_notifiers: ensure range_end() is paired with range_start() If one or more notifiers fails .invalidate_range_start(), invoke .invalidate_range_end() for "all" notifiers. If there are multiple notifiers, those that did not fail are expecting _start() and _end() to be paired, e.g. KVM's mmu_notifier_count would become imbalanced. Disallow notifiers that can fail _start() from implementing _end() so that it's unnecessary to either track which notifiers rejected _start(), or had already succeeded prior to a failed _start(). Note, the existing behavior of calling _start() on all notifiers even after a previous notifier failed _start() was an unintented "feature".=20 Make it canon now that the behavior is depended on for correctness. As of today, the bug is likely benign: 1. The only caller of the non-blocking notifier is OOM kill. 2. The only notifiers that can fail _start() are the i915 and Nouveau drivers. 3. The only notifiers that utilize _end() are the SGI UV GRU driver and KVM. 4. The GRU driver will never coincide with the i195/Nouveau drivers. 5. An imbalanced kvm->mmu_notifier_count only causes soft lockup in the _guest_, and the guest is already doomed due to being an OOM victim. Fix the bug now to play nice with future usage, e.g. KVM has a potential use case for blocking memslot updates in KVM while an invalidation is in-progress, and failure to unblock would result in said updates being blocked indefinitely and hanging. Found by inspection. Verified by adding a second notifier in KVM that periodically returns -EAGAIN on non-blockable ranges, triggering OOM, and observing that KVM exits with an elevated notifier count. Link: https://lkml.kernel.org/r/20210311180057.1582638-1-seanjc@google.com Fixes: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers= ") Signed-off-by: Sean Christopherson Suggested-by: Jason Gunthorpe Reviewed-by: Jason Gunthorpe Cc: David Rientjes Cc: Ben Gardon Cc: Michal Hocko Cc: "J=C3=A9r=C3=B4me Glisse" Cc: Andrea Arcangeli Cc: Johannes Weiner Cc: Dimitri Sivanich Signed-off-by: Andrew Morton --- include/linux/mmu_notifier.h | 10 +++++----- mm/mmu_notifier.c | 23 +++++++++++++++++++++++ 2 files changed, 28 insertions(+), 5 deletions(-) --- a/include/linux/mmu_notifier.h~mm-mmu_notifiers-esnure-range_end-is-pai= red-with-range_start +++ a/include/linux/mmu_notifier.h @@ -169,11 +169,11 @@ struct mmu_notifier_ops { * the last refcount is dropped. * * If blockable argument is set to false then the callback cannot - * sleep and has to return with -EAGAIN. 0 should be returned - * otherwise. Please note that if invalidate_range_start approves - * a non-blocking behavior then the same applies to - * invalidate_range_end. - * + * sleep and has to return with -EAGAIN if sleeping would be required. + * 0 should be returned otherwise. Please note that notifiers that can + * fail invalidate_range_start are not allowed to implement + * invalidate_range_end, as there is no mechanism for informing the + * notifier that its start failed. */ int (*invalidate_range_start)(struct mmu_notifier *subscription, const struct mmu_notifier_range *range); --- a/mm/mmu_notifier.c~mm-mmu_notifiers-esnure-range_end-is-paired-with-ra= nge_start +++ a/mm/mmu_notifier.c @@ -501,10 +501,33 @@ static int mn_hlist_invalidate_range_sta ""); WARN_ON(mmu_notifier_range_blockable(range) || _ret !=3D -EAGAIN); + /* + * We call all the notifiers on any EAGAIN, + * there is no way for a notifier to know if + * its start method failed, thus a start that + * does EAGAIN can't also do end. + */ + WARN_ON(ops->invalidate_range_end); ret =3D _ret; } } } + + if (ret) { + /* + * Must be non-blocking to get here. If there are multiple + * notifiers and one or more failed start, any that succeeded + * start are expecting their end to be called. Do so now. + */ + hlist_for_each_entry_rcu(subscription, &subscriptions->list, + hlist, srcu_read_lock_held(&srcu)) { + if (!subscription->ops->invalidate_range_end) + continue; + + subscription->ops->invalidate_range_end(subscription, + range); + } + } srcu_read_unlock(&srcu, id); =20 return ret; _