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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id B4D56E7BD88 for ; Mon, 16 Feb 2026 10:25:41 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vrvmw-00058b-Cf; Mon, 16 Feb 2026 05:25:10 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vrvmv-00057y-1N for qemu-devel@nongnu.org; Mon, 16 Feb 2026 05:25:09 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vrvmt-0006RZ-DR for qemu-devel@nongnu.org; Mon, 16 Feb 2026 05:25:08 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1771237506; 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=og3R0QAVpEu3XhgTJIvQ+hBOvjqcwqFOe4HoPsi77n4=; b=abmd+pwmFwnrq+tlPlSrPeDPO8smO3jmB6C5K3ElZEqcMI+3U3s2XBA3Veq71QlvJRGnZh 97Jz7WveLvcDlMX+d0cMv73Ut3yJFX/vil8MdL5GjVxbWtsSObIBflEht7XR8ta9MhcWJl JyG9D+jBmmrJJoo9sr3tg9LN6iR1/SM= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-55-aKcJ-0JROU6bPqPHhh3t-Q-1; Mon, 16 Feb 2026 05:25:01 -0500 X-MC-Unique: aKcJ-0JROU6bPqPHhh3t-Q-1 X-Mimecast-MFC-AGG-ID: aKcJ-0JROU6bPqPHhh3t-Q_1771237500 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A5F5C18002C2; Mon, 16 Feb 2026 10:24:59 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.255]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3BBC319560B2; Mon, 16 Feb 2026 10:24:55 +0000 (UTC) Date: Mon, 16 Feb 2026 11:24:53 +0100 From: Kevin Wolf To: Fiona Ebner Cc: qemu-devel@nongnu.org, qemu-stable@nongnu.org, qemu-block@nongnu.org, hreitz@redhat.com, vsementsov@yandex-team.ru, jsnow@redhat.com, jean-louis@dupond.be, dionbosschieter@gmail.com Subject: Re: [PATCH] block/mirror: disable dirty bitmap only after job init Message-ID: References: <20260212120411.369498-1-f.ebner@proxmox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260212120411.369498-1-f.ebner@proxmox.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 12.02.2026 um 13:02 hat Fiona Ebner geschrieben: > Currently, the dirty bitmap is disabled too early and the following > bad scenario is possible: > > 1. Dirty bitmap is disabled in mirror_start_job() > 2. Some request are started in mirror_top_bs while s->job == NULL > 3. mirror_dirty_init() -> bdrv_co_is_allocated_above() runs and because > the request hasn't completed yet, the block isn't allocated > 4. The request completes, still sees s->job == NULL and skips the > bitmap, and nothing else will mark it dirty either > > One ingredient is that mirror_top_opaque->job is only set after the > job is fully initialized. For the rationale, see commit 32125b1460 > ("mirror: Fix access of uninitialised fields during start"). > > Disabling the dirty bitmap is safe once bdrv_mirror_top_do_write() > sees that the job is set, because then: > > 1. When not using MIRROR_COPY_MODE_WRITE_BLOCKING, the dirty bitmap > will be set by bdrv_mirror_top_do_write(). > > 2. When using MIRROR_COPY_MODE_WRITE_BLOCKING, writes will be done > synchronously to the target. > > At least with virtio-blk using iothread-vq-mapping, mirror_run() and > bdrv_mirror_top_do_write() might be called in different threads. > bdrv_disable_dirty_bitmap() acquires and releases the dirty bitmap > mutex, so the memory is synchronized between threads. > > Many thanks to Kevin Wolf for discussing the issue and solutions with > me! [0] > > [0]: https://lore.kernel.org/qemu-devel/4853b0e5-8ec3-41e9-9a53-b1912b8e4449@dupond.be/T/ > > Cc: qemu-stable@nongnu.org > Closes: https://gitlab.com/qemu-project/qemu/-/issues/3273 > Signed-off-by: Fiona Ebner > --- > block/mirror.c | 21 +++++++++++++++------ > 1 file changed, 15 insertions(+), 6 deletions(-) > > diff --git a/block/mirror.c b/block/mirror.c > index b344182c74..eadd4501e8 100644 > --- a/block/mirror.c > +++ b/block/mirror.c > @@ -1123,6 +1123,21 @@ static int coroutine_fn mirror_run(Job *job, Error **errp) > */ > mirror_top_opaque->job = s; > > + /* > + * Disabling the dirty bitmap is safe once bdrv_mirror_top_do_write() sees > + * that the job is set, because then: > + * > + * 1. When not using MIRROR_COPY_MODE_WRITE_BLOCKING, the dirty bitmap will > + * be set by bdrv_mirror_top_do_write(). > + * > + * 2. When using MIRROR_COPY_MODE_WRITE_BLOCKING, writes will be done > + * synchronously to the target. > + * > + * bdrv_disable_dirty_bitmap() acquires and releases the dirty bitmap mutex, > + * so the memory is synchronized between threads. > + */ > + bdrv_disable_dirty_bitmap(s->dirty_bitmap); > + > assert(!s->dbi); > s->dbi = bdrv_dirty_iter_new(s->dirty_bitmap); > for (;;) { > @@ -2014,12 +2029,6 @@ static BlockJob *mirror_start_job( > goto fail; > } > > - /* > - * The dirty bitmap is set by bdrv_mirror_top_do_write() when not in active > - * mode. > - */ > - bdrv_disable_dirty_bitmap(s->dirty_bitmap); > - > bdrv_graph_wrlock_drained(); > ret = block_job_add_bdrv(&s->common, "source", bs, 0, > BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE | The thing I meant in the other thread is if we don't need something like this additionally: diff --git a/block/mirror.c b/block/mirror.c index b344182c747..159954158ba 100644 --- a/block/mirror.c +++ b/block/mirror.c @@ -1672,9 +1672,17 @@ bdrv_mirror_top_do_write(BlockDriverState *bs, MirrorMethod method, abort(); } - if (!copy_to_target && s->job && s->job->dirty_bitmap) { + if (!copy_to_target) { qatomic_set(&s->job->actively_synced, false); - bdrv_set_dirty_bitmap(s->job->dirty_bitmap, offset, bytes); + if (s->job && s->job->dirty_bitmap) { + bdrv_set_dirty_bitmap(s->job->dirty_bitmap, offset, bytes); + } else { + /* + * Avoid race in the case that mirror_run() disables the bitmap + * between here and bdrv_co_write_req_finish(). + */ + bdrv_set_dirty(bs, offset, bytes); + } } if (ret < 0) { By the way, can it ever happen that s->job && !s->job->dirty_bitmap or is that second part of the condition redundant? Kevin