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=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 E9AA2C56201 for ; Tue, 24 Nov 2020 09:18:38 +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 C6F202073C for ; Tue, 24 Nov 2020 09:18:37 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Kwve2/qR" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C6F202073C 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]:49384 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1khUT6-0002lj-Nw for qemu-devel@archiver.kernel.org; Tue, 24 Nov 2020 04:18:36 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:43714) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1khUS8-0002Es-Jy for qemu-devel@nongnu.org; Tue, 24 Nov 2020 04:17:36 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:27849) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1khUS5-0003VK-Kj for qemu-devel@nongnu.org; Tue, 24 Nov 2020 04:17:35 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606209452; 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: in-reply-to:in-reply-to:references:references; bh=kgduawD1Mhmp8e6ovymPM2l6a6U4/pJ77FYki16+7rE=; b=Kwve2/qR8EZf1ILb9YKbKdmSZIWMt3y4ejdeERyTyCI4Ju2sNTmKqYMjue8TLzgQPFnoYy /K06noysyKn+8lmH9gJ4GervB+p2DjOkOGCG49+av6G7rQcxty6bp9lGvyPWoS4kcvf9Qp DdrPW3f5xVWeLLi0bkXNTdpXFcEDj9Q= 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-323-n98BvP6KPvW_z-7s-bHWng-1; Tue, 24 Nov 2020 04:17:30 -0500 X-MC-Unique: n98BvP6KPvW_z-7s-bHWng-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 86D503E75D; Tue, 24 Nov 2020 09:17:28 +0000 (UTC) Received: from merkur.fritz.box (ovpn-114-111.ams2.redhat.com [10.36.114.111]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 779F219C71; Tue, 24 Nov 2020 09:17:25 +0000 (UTC) Date: Tue, 24 Nov 2020 10:17:23 +0100 From: Kevin Wolf To: Maxim Levitsky Subject: Re: [PATCH 1/1] Fix qcow2 corruption on discard Message-ID: <20201124091723.GA22385@merkur.fritz.box> References: <20201123154929.330338-1-mlevitsk@redhat.com> <20201123154929.330338-2-mlevitsk@redhat.com> <20201123173853.GE5317@merkur.fritz.box> <776008a350e47a33adbe659aa4ba106b6a2daf5f.camel@redhat.com> MIME-Version: 1.0 In-Reply-To: <776008a350e47a33adbe659aa4ba106b6a2daf5f.camel@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=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: vsementsov@virtuozzo.com, Alberto Garcia , qemu-block@nongnu.org, zhang_youjia@126.com, qemu-devel@nongnu.org, Max Reitz , andrey.shinkevich@virtuozzo.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 23.11.2020 um 19:11 hat Maxim Levitsky geschrieben: > On Mon, 2020-11-23 at 18:38 +0100, Kevin Wolf wrote: > > Am 23.11.2020 um 16:49 hat Maxim Levitsky geschrieben: > > > Commit 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()") > > > introduced a subtle change to code in zero_in_l2_slice: > > > > > > It swapped the order of > > > > > > 1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > > > 2. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO); > > > 3. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST); > > > > > > To > > > > > > 1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > > > 2. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST); > > > 3. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO); > > > > > > It seems harmless, however the call to qcow2_free_any_clusters > > > can trigger a cache flush which can mark the L2 table as clean, > > > and assuming that this was the last write to it, > > > a stale version of it will remain on the disk. > > > > Do you have more details on this last paragraph? I'm trying to come up > > with a reproducer, but I don't see how qcow2_free_any_clusters() could > > flush the L2 table cache. (It's easy to get it to flush the refcount > > block cache, but that's useless for a reproducer.) > > > > The only way I see to flush any cache with it is in update_refcount() > > the qcow2_cache_set_dependency() call. This will always flush the cache > > that the L2 cache depends on - which will never be the L2 cache itself, > > but always either the refcount cache or nothing. > > > > There are more options in alloc_refcount_block() if we're allocating a > > new refcount block, but in the context of freeing clusters we'll never > > need to do that. > > > > Whatever I tried, at the end of zero_in_l2_slice(), I have a dirty L2 > > table and a dirty refcount block in the cache, with a dependency that > > makes sure that the L2 table will be written out first. > > > > If you don't have the information yet, can you try to debug your manual > > reproducer a bit more to find out how this happens? > I'll do this tomorrow. As the last RC for 5.2 is today, I will send a v2 that changes the fix to restore the original order. We can then continue work to find a minimal reproducer and merge the test case in the early 6.0 cycle. Kevin