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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 010B0C76186 for ; Mon, 29 Jul 2019 05:55:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D537F2070B for ; Mon, 29 Jul 2019 05:55:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726851AbfG2FzD (ORCPT ); Mon, 29 Jul 2019 01:55:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43666 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725988AbfG2FzD (ORCPT ); Mon, 29 Jul 2019 01:55:03 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3B5AF30BD1C0; Mon, 29 Jul 2019 05:55:02 +0000 (UTC) Received: from [10.72.12.53] (ovpn-12-53.pek2.redhat.com [10.72.12.53]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3C5D55D9C3; Mon, 29 Jul 2019 05:54:50 +0000 (UTC) Subject: Re: WARNING in __mmdrop To: "Michael S. Tsirkin" Cc: syzbot , aarcange@redhat.com, akpm@linux-foundation.org, christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com, elena.reshetova@intel.com, guro@fb.com, hch@infradead.org, james.bottomley@hansenpartnership.com, jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, luto@amacapital.net, mhocko@suse.com, mingo@kernel.org, namit@vmware.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, wad@chromium.org References: <55e8930c-2695-365f-a07b-3ad169654d28@redhat.com> <20190725042651-mutt-send-email-mst@kernel.org> <84bb2e31-0606-adff-cf2a-e1878225a847@redhat.com> <20190725092332-mutt-send-email-mst@kernel.org> <11802a8a-ce41-f427-63d5-b6a4cf96bb3f@redhat.com> <20190726074644-mutt-send-email-mst@kernel.org> <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com> <20190726082837-mutt-send-email-mst@kernel.org> <20190726094756-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <0792ee09-b4b7-673c-2251-e5e0ce0fbe32@redhat.com> Date: Mon, 29 Jul 2019 13:54:49 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190726094756-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Mon, 29 Jul 2019 05:55:02 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/7/26 下午9:49, Michael S. Tsirkin wrote: >>> Ok, let me retry if necessary (but I do remember I end up with deadlocks >>> last try). >> Ok, I play a little with this. And it works so far. Will do more testing >> tomorrow. >> >> One reason could be I switch to use get_user_pages_fast() to >> __get_user_pages_fast() which doesn't need mmap_sem. >> >> Thanks > OK that sounds good. If we also set a flag to make > vhost_exceeds_weight exit, then I think it will be all good. After some experiments, I came up two methods: 1) switch to use vq->mutex, then we must take the vq lock during range checking (but I don't see obvious slowdown for 16vcpus + 16queues). Setting flags during weight check should work but it still can't address the worst case: wait for the page to be swapped in. Is this acceptable? 2) using current RCU but replace synchronize_rcu() with vhost_work_flush(). The worst case is the same as 1) but we can check range without holding any locks. Which one did you prefer? Thanks