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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 E1D5DC433FF for ; Sun, 4 Aug 2019 00:14:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B5ED52087C for ; Sun, 4 Aug 2019 00:14:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca header.b="lAi4L6Z1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729330AbfHDAOE (ORCPT ); Sat, 3 Aug 2019 20:14:04 -0400 Received: from mail-qt1-f196.google.com ([209.85.160.196]:35044 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729259AbfHDAOE (ORCPT ); Sat, 3 Aug 2019 20:14:04 -0400 Received: by mail-qt1-f196.google.com with SMTP id d23so77718181qto.2 for ; Sat, 03 Aug 2019 17:14:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ziepe.ca; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=WqP5IEn9c1KXpbjHniLk8HqGR3ikHJzys8MEePZlMFc=; b=lAi4L6Z1XyIiSM5lL1y3xq2yW7AhMhA5cc/2JWhK4Uq58oYTXRY+SEfOyGjKf6kkCb EPC9/wo5tKQKNgcCq7J5u94OLKKBkYAT9e7xtRKka5BNmKvZWgWXct5IFZiXhtUBed8m rk3hbmVvbY+x2rSZElIcRMSuJak9sbEMLouEsX5kctrioFW3dP5ZTCHUO7a88gwJF86c Kw4vjibh/yQ70e+pFUaKMHvh1MOZwG0UveYwUOrZxaDK9+spjQNaOE305bhzgGLhXZaG 1qCnoPNDxKWD7uKl46B5GsKjuaJdx75qFBHdD8Xa8h8Svul5AL5C3uyvDl1nKlcQKCkP pPcQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=WqP5IEn9c1KXpbjHniLk8HqGR3ikHJzys8MEePZlMFc=; b=ZSK5fRm7DEoMFWxTXE/6ELaE/1CTPA0EzaD5VBi91YKE2wlIZu8dVEfSA49V9UVw+1 0W5zHF0thIvthpIBfzvJcQgah+7y36Bfkwo+5BZtMgf2BX28w4vIhXELcPSV6voA1LbY FSgy+TjahloYFnDvbcG14D4hq+l6duR7ftVx20HNFnz4SxJSiK84b2AR0U8YUrDFBRX4 nXlyWzXaIHRramUCna46ADV/0IpOPNc999UGwgdV4buA1a4v/OXmgnK/IsykbB+iTJbe WteOu0WQOFfFRrCsDDxNBy8nDDfJ4I7OnJO4+QNjLVRa6C3K7+/Syy694IVHOV+5cwjr xhRw== X-Gm-Message-State: APjAAAUJnij8e8mGXAbv9r4U3EGKSm+rCo/lLX8XtlD7YtPh2oy6rKK6 A5zlSU6Ksw1FiPnGLF8/jq9NQQ== X-Google-Smtp-Source: APXvYqzsyyLWq0jXkEok0bUf3FHiInYAhbn8RrkmZKAfLQFhGrLDITCXgW1cHLxjIP+uPvSHKyq3TA== X-Received: by 2002:a05:6214:1312:: with SMTP id a18mr103640128qvv.241.1564877642991; Sat, 03 Aug 2019 17:14:02 -0700 (PDT) Received: from ziepe.ca (hlfxns017vw-156-34-55-100.dhcp-dynamic.fibreop.ns.bellaliant.net. [156.34.55.100]) by smtp.gmail.com with ESMTPSA id g35sm42675590qtg.92.2019.08.03.17.14.01 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sat, 03 Aug 2019 17:14:01 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hu49w-0006gS-Bm; Sat, 03 Aug 2019 21:14:00 -0300 Date: Sat, 3 Aug 2019 21:14:00 -0300 From: Jason Gunthorpe To: "Michael S. Tsirkin" Cc: Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker Message-ID: <20190804001400.GA25543@ziepe.ca> References: <20190731123935.GC3946@ziepe.ca> <7555c949-ae6f-f105-6e1d-df21ddae9e4e@redhat.com> <20190731193057.GG3946@ziepe.ca> <20190801141512.GB23899@ziepe.ca> <42ead87b-1749-4c73-cbe4-29dbeb945041@redhat.com> <20190802124613.GA11245@ziepe.ca> <20190802100414-mutt-send-email-mst@kernel.org> <20190802172418.GB11245@ziepe.ca> <20190803172944-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190803172944-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 03, 2019 at 05:36:13PM -0400, Michael S. Tsirkin wrote: > On Fri, Aug 02, 2019 at 02:24:18PM -0300, Jason Gunthorpe wrote: > > On Fri, Aug 02, 2019 at 10:27:21AM -0400, Michael S. Tsirkin wrote: > > > On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote: > > > > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: > > > > > > This must be a proper barrier, like a spinlock, mutex, or > > > > > > synchronize_rcu. > > > > > > > > > > > > > > > I start with synchronize_rcu() but both you and Michael raise some > > > > > concern. > > > > > > > > I've also idly wondered if calling synchronize_rcu() under the various > > > > mm locks is a deadlock situation. > > > > > > > > > Then I try spinlock and mutex: > > > > > > > > > > 1) spinlock: add lots of overhead on datapath, this leads 0 performance > > > > > improvement. > > > > > > > > I think the topic here is correctness not performance improvement > > > > > > The topic is whether we should revert > > > commit 7f466032dc9 ("vhost: access vq metadata through kernel virtual address") > > > > > > or keep it in. The only reason to keep it is performance. > > > > Yikes, I'm not sure you can ever win against copy_from_user using > > mmu_notifiers? > > Ever since copy_from_user started playing with flags (for SMAP) and > added speculation barriers there's a chance we can win by accessing > memory through the kernel address. You think copy_to_user will be more expensive than the minimum two atomics required to synchronize with another thread? > > Also, why can't this just permanently GUP the pages? In fact, where > > does it put_page them anyhow? Worrying that 7f466 adds a get_user page > > but does not add a put_page?? You didn't answer this.. Why not just use GUP? Jason