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=-5.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 C7D7AC07E95 for ; Tue, 20 Jul 2021 14:33:46 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 60A7960FE7 for ; Tue, 20 Jul 2021 14:33:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 60A7960FE7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:51612 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1m5qob-0001ej-HI for qemu-devel@archiver.kernel.org; Tue, 20 Jul 2021 10:33:45 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:42418) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m5qnN-0007dP-6v for qemu-devel@nongnu.org; Tue, 20 Jul 2021 10:32:29 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:42888) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m5qnL-0007t9-4o for qemu-devel@nongnu.org; Tue, 20 Jul 2021 10:32:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1626791539; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=VJKAhG7+4ni6ArQPJPutIR8ForI6fcNkQN+9/ndFybA=; b=VXCgBsg20roy4yXFWmsToSeIs0VqvEZn6lwON6lI+Ine/JqmRja+WeniQV9eZQBpEoQzlc 7zZ54wLEkx5csMc3AfYq1yF6Meq9XzqAtMqMVPe+QLpAIpKZ7fWiqYKEZyEx4OXAIcYwaE H4UuimHO+p5DZtRmg7pMjlbQoeLZj0E= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-37-cixJGfErM0ePthnwWZwWaA-1; Tue, 20 Jul 2021 10:32:16 -0400 X-MC-Unique: cixJGfErM0ePthnwWZwWaA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A773250758; Tue, 20 Jul 2021 14:32:15 +0000 (UTC) Received: from redhat.com (ovpn-113-31.ams2.redhat.com [10.36.113.31]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5CC4E369A; Tue, 20 Jul 2021 14:32:02 +0000 (UTC) Date: Tue, 20 Jul 2021 15:31:59 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: David Hildenbrand Subject: Re: [PATCH v1 3/3] util/oslib-posix: Support concurrent os_mem_prealloc() invocation Message-ID: References: <20210714112306.67793-1-david@redhat.com> <20210714112306.67793-4-david@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.474, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Pankaj Gupta , Eduardo Habkost , "Michael S. Tsirkin" , qemu-devel@nongnu.org, "Dr . David Alan Gilbert" , Paolo Bonzini , Igor Mammedov , Marek Kedzierski Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Jul 20, 2021 at 04:27:32PM +0200, David Hildenbrand wrote: > On 20.07.21 16:22, Daniel P. Berrangé wrote: > > On Wed, Jul 14, 2021 at 01:23:06PM +0200, David Hildenbrand wrote: > > > Add a mutext to protect the SIGBUS case, as we cannot mess concurrently > > > > typo s/mutext/mutex/ > > > > > with the sigbus handler and we have to manage the global variable > > > sigbus_memset_context. The MADV_POPULATE_WRITE path can run > > > concurrently. > > > > > > Note that page_mutex and page_cond are shared between concurrent > > > invocations, which shouldn't be a problem. > > > > > > This is a preparation for future virtio-mem prealloc code, which will call > > > os_mem_prealloc() asynchronously from an iothread when handling guest > > > requests. > > > > Hmm, I'm wondering how the need to temporarily play with SIGBUS > > at runtime for mem preallocation will interact with the SIGBUS > > handler installed by softmmu/cpus.c. > > That's exactly why I came up with MADV_POPULATE_WRITE, to avoid having to > mess with different kinds of sigbus at the same time. You can only get it > wrong. > > > > > The SIGBUS handler the preallocation code is installed just > > blindly assumes the SIGBUS is related to the preallocation > > work being done. This is a fine assumption during initially > > startup where we're single threaded and not running guest > > CPUs. I'm less clear on whether that's a valid assumption > > at runtime once guest CPUs are running. > > I assume it's quite broken, for example, already when hotplugging a DIMM and > prallocating memory for the memory backend. > > > > > If the sigbus_handler method in softmmu/cpus.c is doing > > something important for QEMU, then why is it ok for us to > > periodically disable that handler and replace it with > > something else that takes a completely different action ? > > I don't think it is ok. I assume it has been broken for a long time, just > nobody ever ran into that race. > > > > > Of course with the madvise impl we're bypassing the SIGBUS > > dance entirely. This is good for people with new kernels, > > but is this SIGBUS stuff safe for older kernels ? > > It remains broken with old kernels I guess. There isn't too much that we can > do: disabling prealloc=on once the VM is running breaks existing use cases. Ok, while refactoring this, could you add a scary warning next to the sigaction calls mentioning that this code is not likely to play well with qemu's other handling of sigbus, as a reminder to future reviewers. > Fortunately, running into that race seems to be rare, at least I never hear > reports. The failure mode is likely to be silent or easily mis-interpreted Is there any value in emitting a one-time per process warning message on stderr if we take the old codepath post-startup ? Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|