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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, 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 5721FC433E1 for ; Tue, 30 Mar 2021 12:53:00 +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 DB432619B1 for ; Tue, 30 Mar 2021 12:52:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DB432619B1 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]:41988 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lRDre-0003s8-R4 for qemu-devel@archiver.kernel.org; Tue, 30 Mar 2021 08:52:58 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60792) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lRDpy-0002Nk-VQ for qemu-devel@nongnu.org; Tue, 30 Mar 2021 08:51:14 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:24134) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lRDpx-000126-1k for qemu-devel@nongnu.org; Tue, 30 Mar 2021 08:51:14 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617108672; h=from:from: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=trGN9gMgRGPRLq/gri4GbOLSH8LdtJEdsSIuiPHH1vA=; b=LSiH7T0kdj9ctKT2gtf97iYNWh1uy0AxkgOFCc/GfVAffZw6mrq0sBVRWgBs28l/ZfNLCl FHnnAoyTpnlaTQM1LvlSQPgB11qMpb6EJmoghQjqDC9iIB00Tp0cYOGu2JFaR9Ra2EMqmK dl32kM9TK6A52D9QVTUY4IzAcK6UyDE= 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-297-N2dN9Zs7N52AfSHoYAQ_5A-1; Tue, 30 Mar 2021 08:51:10 -0400 X-MC-Unique: N2dN9Zs7N52AfSHoYAQ_5A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 23213180FCB7; Tue, 30 Mar 2021 12:51:09 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-113-220.ams2.redhat.com [10.36.113.220]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D20C560CD0; Tue, 30 Mar 2021 12:51:07 +0000 (UTC) Subject: Re: [PATCH v4 for-6.0? 0/3] qcow2: fix parallel rewrite and discard (rw-lock) To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210319100811.342464-1-vsementsov@virtuozzo.com> <166496e1-f855-5813-2fe8-88035fb85a26@virtuozzo.com> From: Max Reitz Message-ID: <80e4838f-772b-28e1-3bdb-10a5d7e72ec0@redhat.com> Date: Tue, 30 Mar 2021 14:51:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <166496e1-f855-5813-2fe8-88035fb85a26@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, 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: , Cc: kwolf@redhat.com, den@openvz.org, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 30.03.21 12:51, Vladimir Sementsov-Ogievskiy wrote: > 30.03.2021 12:49, Max Reitz wrote: >> On 25.03.21 20:12, Vladimir Sementsov-Ogievskiy wrote: >>> ping. Do we want it for 6.0? >> >> I’d rather wait.  I think the conclusion was that guests shouldn’t hit >> this because they serialize discards? > > I think, that we never had bugs, so we of course can wait. > >> >> There’s also something Kevin wrote on IRC a couple of weeks ago, for >> which I had hoped he’d sent an email but I don’t think he did, so I’ll >> try to remember and paraphrase as well as I can... >> >> He basically asked whether it wouldn’t be conceptually simpler to take >> a reference to some cluster in get_cluster_offset() and later release >> it with a to-be-added put_cluster_offset(). >> >> He also noted that reading is problematic, too, because if you read a >> discarded and reused cluster, this might result in an information leak >> (some guest application might be able to read data it isn’t allowed to >> read); that’s why making get_cluster_offset() the point of locking >> clusters against discarding would be better. > > Yes, I thought about read too, (RFCed in cover letter of [PATCH v5 0/6] > qcow2: fix parallel rewrite and discard (lockless)) > >> >> This would probably work with both of your solutions.  For the >> in-memory solutions, you’d take a refcount to an actual cluster; in >> the CoRwLock solution, you’d take that lock. >> >> What do you think? >> > > Hmm. What do you mean? Just rename my qcow2_inflight_writes_inc() and > qcow2_inflight_writes_dec() to > get_cluster_offset()/put_cluster_offset(), to make it more native to use > for read operations as well? Hm. Our discussion wasn’t so detailed. I interpreted it to mean all qcow2 functions that find an offset to a qcow2 cluster, namely qcow2_get_host_offset(), qcow2_alloc_host_offset(), and qcow2_alloc_compressed_cluster_offset(). When those functions return an offset (in)to some cluster, that cluster (or the image as a whole) should be locked against discards. Every offset received this way would require an accompanying qcow2_put_host_offset(). > Or to update any kind of "getting cluster offset" in the whole qcow2 > driver to take a kind of "dynamic reference count" by > get_cluster_offset() and then call corresponding put() somewhere? In > this case I'm afraid it's a lot more work.. Hm, really? I would have assumed we need to do some locking in all functions that get a cluster offset this way, so it should be less work to take the lock in the functions they invoke to get the offset. > It would be also the problem > that a lot of paths in qcow2 are not in coroutine and don't even take > s->lock when they actually should. I’m not sure what you mean here, because all functions that invoke any of the three functions I listed above are coroutine_fns (or, well, I didn’t look it up, but they all have *_co_* in their name). > This will also mean that we do same > job as normal qcow2 refcounts already do: no sense in keeping additional > "dynamic refcount" for L2 table cluster while reading it, as we already > have non-zero qcow2 normal refcount for it.. I’m afraid I don’t understand how normal refcounts relate to this. For example, qcow2_get_host_offset() doesn’t touch refcounts at all. Max