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 DE6DEE81A44 for ; Mon, 16 Feb 2026 16:39:52 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vs1co-00062p-Ou; Mon, 16 Feb 2026 11:39:06 -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 1vs1cm-00062K-MS for qemu-devel@nongnu.org; Mon, 16 Feb 2026 11:39:04 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vs1cY-00066x-Ag for qemu-devel@nongnu.org; Mon, 16 Feb 2026 11:39:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1771259928; 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=HpkCBfYtl8QocC2bsxqsbI3sWgiULrIYgM/ZiGTqRx0=; b=VSfIhTtsdql9gQXNxUPCYOSTo+Duerx+Fpf3rv3wLH2ZNbLrIOtmzFKVrmyYhleepdRNxp NImIm9p1lOtIzdqoRcAUYZVkfpmJAPC9IUP0Q6OZTbT7vZwQ/qAZQ1DZdOjq4zOZOXcNZl ez8LxOePwmUkLRFfoTfy8OvzSzJsEs0= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-352-l41EEGKaNc-z0_EQcP74AQ-1; Mon, 16 Feb 2026 11:38:44 -0500 X-MC-Unique: l41EEGKaNc-z0_EQcP74AQ-1 X-Mimecast-MFC-AGG-ID: l41EEGKaNc-z0_EQcP74AQ_1771259923 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3561419560B7; Mon, 16 Feb 2026 16:38:42 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.255]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id C31F518003F6; Mon, 16 Feb 2026 16:38:38 +0000 (UTC) Date: Mon, 16 Feb 2026 17:38:36 +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> <66c3fca8-0f64-4cec-a2a9-5959ef67a251@proxmox.com> <5f336c06-c675-4d21-8c1c-e1a1ebe8e09f@proxmox.com> <811bbd59-df4e-4a60-adba-61cd2ef3016c@proxmox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <811bbd59-df4e-4a60-adba-61cd2ef3016c@proxmox.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.133.124; 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_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 16.02.2026 um 17:12 hat Fiona Ebner geschrieben: > Am 16.02.26 um 4:31 PM schrieb Kevin Wolf: > > Am 16.02.2026 um 14:01 hat Fiona Ebner geschrieben: > >> Am 16.02.26 um 1:48 PM schrieb Fiona Ebner: > >>> Am 16.02.26 um 11:25 AM schrieb Kevin Wolf: > >>>> 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) { > >>> > >>> You're right! It's not enough to ensure that the job is set before the > >>> bitmap is disabled, because both could happen after the check for job here. > >>> > >>> Isn't this change enough by itself? After the change, > >>> bdrv_mirror_top_do_write() ensures that: > >>> > >>> 1. When copy_to_target == true, the write is done synchronously to the > >>> target, no need to set the dirty bitmap. > >>> > >>> 2. When copy_to_target == false, the dirty bitmap is set. > >>> > >>> So it doesn't really matter at which point the dirty bitmap is disabled? > >>> Or am I missing something again? > >> > >> Ah right, bdrv_set_dirty() only sets it if still enabled :) > > > > Yes, exactly. > > > > If that's still too confusing, how about just giving mirror_top_bs > > access to its dirty bitmap right from the start while it's drained? I > > didn't try to reproduce the bug with it yet, though. > > You beat me to it :) Was thinking about this too, inspired by your > initial suggestion for the alternate approach to avoid relying on the > block layer for dirty tracking, but mine ended up needlessly involved, > because I removed the dirty_bitmap from the MirrorBlockJob object and > had to add a helper function for getting the dirty bitmap back from the > job, which is less than ideal :P > > > > Kevin > > > > diff --git a/block/mirror.c b/block/mirror.c > > index b344182c747..c11aca1366c 100644 > > --- a/block/mirror.c > > +++ b/block/mirror.c > > @@ -99,6 +99,7 @@ typedef struct MirrorBlockJob { > > > > typedef struct MirrorBDSOpaque { > > MirrorBlockJob *job; > > + BdrvDirtyBitmap *dirty_bitmap; > > bool stop; > > bool is_commit; > > } MirrorBDSOpaque; > > @@ -1672,9 +1673,9 @@ bdrv_mirror_top_do_write(BlockDriverState *bs, MirrorMethod method, > > abort(); > > } > > > > - if (!copy_to_target && s->job && s->job->dirty_bitmap) { > > + if (!copy_to_target) { > > I put an assert(s->dirty_bitmap) here, but maybe it's not worth it? The difference is SIGABRT here vs. SIGSEGV two lines later. I don't think it matters much, either way is fine with me. > > qatomic_set(&s->job->actively_synced, false); > > - bdrv_set_dirty_bitmap(s->job->dirty_bitmap, offset, bytes); > > + bdrv_set_dirty_bitmap(s->dirty_bitmap, offset, bytes); > > } > > > > if (ret < 0) { > > @@ -1901,13 +1902,28 @@ static BlockJob *mirror_start_job( > > I created and disabled the dirty bitmap here already before the drained > section. It seems slightly more readable. Do you see any downside to that? Hm... I think it's okay in practice because a drain comes after it, so while there may be a race initially, we don't care when exactly we start tracking as long as it's before checking the block status. But that's a bit subtle. We could keep creating the bitmap outside of the drain if that improves things, but I would leave bdrv_disable_dirty_bitmap() inside the drained section just to be very clear that there can't be a race with a request. > > > > bdrv_drained_begin(bs); > > ret = bdrv_append(mirror_top_bs, bs, errp); > > - bdrv_drained_end(bs); > > - > > if (ret < 0) { > > + bdrv_drained_end(bs); > > bdrv_unref(mirror_top_bs); > > return NULL; > > } > > > > + bs_opaque->dirty_bitmap = bdrv_create_dirty_bitmap(mirror_top_bs, > > + granularity, > > + NULL, errp); > > + if (!bs_opaque->dirty_bitmap) { > > + bdrv_drained_end(bs); > > + bdrv_unref(mirror_top_bs); > > + return NULL; > > + } > > + > > + /* > > + * The dirty bitmap is set by bdrv_mirror_top_do_write() when not in active > > + * mode. > > + */ > > This comment could become: > > + /* > + * Disabling the dirty bitmap is safe, because: > + * > + * 1. If should_copy_to_target() == true, writes will be done > synchronously > + * to the target, no need to set the bitmap. > + * > + * 2. If should_copy_to_target() == false, the dirty bitmap will be > set by > + * bdrv_mirror_top_do_write(). > + */ Feels verbose to me because the implications of the existing comment seem obvious to me, but if you think it's useful, it probably isn't as obvious as I thought. > > + bdrv_disable_dirty_bitmap(bs_opaque->dirty_bitmap); > > + bdrv_drained_end(bs); > > + > > /* Make sure that the source is not resized while the job is running */ > > s = block_job_create(job_id, driver, NULL, mirror_top_bs, > > BLK_PERM_CONSISTENT_READ, > > @@ -2002,24 +2018,13 @@ static BlockJob *mirror_start_job( > > s->base_overlay = bdrv_find_overlay(bs, base); > > s->granularity = granularity; > > s->buf_size = ROUND_UP(buf_size, granularity); > > + s->dirty_bitmap = bs_opaque->dirty_bitmap; > > s->unmap = unmap; > > if (auto_complete) { > > s->should_complete = true; > > } > > bdrv_graph_rdunlock_main_loop(); > > > > - s->dirty_bitmap = bdrv_create_dirty_bitmap(s->mirror_top_bs, granularity, > > - NULL, errp); > > - if (!s->dirty_bitmap) { > > - 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 | > > @@ -2099,9 +2104,6 @@ fail: > > g_free(s->replaces); > > blk_unref(s->target); > > bs_opaque->job = NULL; > > - if (s->dirty_bitmap) { > > - bdrv_release_dirty_bitmap(s->dirty_bitmap); > > - } > > job_early_fail(&s->common.job); > > } > > > > @@ -2115,6 +2117,7 @@ fail: > > bdrv_graph_wrunlock(); > > bdrv_drained_end(bs); > > > > + bdrv_release_dirty_bitmap(bs_opaque->dirty_bitmap); > > bdrv_unref(mirror_top_bs); > > > > return NULL; Kevin