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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]:47336 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1khGMw-0005GU-F6 for qemu-devel@archiver.kernel.org; Mon, 23 Nov 2020 13:15:18 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:54706) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1khGJu-0002ln-Fo for qemu-devel@nongnu.org; Mon, 23 Nov 2020 13:12:10 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:32976) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1khGJs-0008J5-Bh for qemu-devel@nongnu.org; Mon, 23 Nov 2020 13:12:09 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606155126; 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=iRsWDADpHtNKAP6L/gK+z4PZjiFnpgyt2TYRo0TRgwk=; b=ZdEbQ92lbm/goGur82wQDcItiUI0+AjlJm2rUozfwbqIBUS7sP/zxQrpQfh6cG74WpXW4u ZSdqKT5ZeJlmxqBvKNZDHoaJqejHY4UEvdhwGarjq+zYhjcj5N+RbvLLYLWDKrq4ytA6kW EAPmSvOPcLS62zj3FAL8Sz0qWFb4o8g= 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-482-V6lqNApBOGSInXhfv086BA-1; Mon, 23 Nov 2020 13:11:57 -0500 X-MC-Unique: V6lqNApBOGSInXhfv086BA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7877F18C89C4; Mon, 23 Nov 2020 18:11:56 +0000 (UTC) Received: from starship (unknown [10.35.206.216]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9A6655D705; Mon, 23 Nov 2020 18:11:52 +0000 (UTC) Message-ID: <776008a350e47a33adbe659aa4ba106b6a2daf5f.camel@redhat.com> Subject: Re: [PATCH 1/1] Fix qcow2 corruption on discard From: Maxim Levitsky To: Kevin Wolf Date: Mon, 23 Nov 2020 20:11:51 +0200 In-Reply-To: <20201123173853.GE5317@merkur.fritz.box> References: <20201123154929.330338-1-mlevitsk@redhat.com> <20201123154929.330338-2-mlevitsk@redhat.com> <20201123173853.GE5317@merkur.fritz.box> User-Agent: Evolution 3.36.3 (3.36.3-1.fc32) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mlevitsk@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=mlevitsk@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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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" 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. Best regards, Maxim Levitsky > > Kevin > > > Now we have a valid L2 entry pointing to a freed cluster. Oops. > > > > Fixes: 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()") > > Signed-off-by: Maxim Levitsky > > --- > > block/qcow2-cluster.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > > index 485b4cb92e..267b46a4ca 100644 > > --- a/block/qcow2-cluster.c > > +++ b/block/qcow2-cluster.c > > @@ -2010,11 +2010,11 @@ static int zero_in_l2_slice(BlockDriverState *bs, uint64_t offset, > > continue; > > } > > > > - qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > > if (unmap) { > > qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST); > > } > > set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry); > > + qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice); > > if (has_subclusters(s)) { > > set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap); > > } > > -- > > 2.26.2 > >