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bh=UuaeK2f9mN8U9L/XEH6DS6+tp2HVIMqwEJqwoyIfvCE=; b=cls1JO6YBGjvMG+NogNdlZJn7wRDqvfZfioi49LkfP9b/uLAPSizjRg527YehZ2QzNninl TwhBT+hebs1rcvzi+H1lbdi5thOBwlyoPCqm0tXWtfOYcAQr6xMJSCD9mTmuYs4b+2E3d2 an2N0SP698Bav8J3ZFv8pz+evus6ZtQ= 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-385-LPNpn2N2NAifUN2qFNlrCw-1; Wed, 05 May 2021 12:25:37 -0400 X-MC-Unique: LPNpn2N2NAifUN2qFNlrCw-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 6FC84107ACC7; Wed, 5 May 2021 16:25:36 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-115-153.ams2.redhat.com [10.36.115.153]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CCEF719C46; Wed, 5 May 2021 16:25:34 +0000 (UTC) Subject: Re: [PATCH] block/snapshot: Clarify goto fallback behavior To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210503095418.31521-1-mreitz@redhat.com> <29db0d1f-2d60-7df6-e6d0-97b89cf636ef@virtuozzo.com> From: Max Reitz Message-ID: <11eaa840-a882-3ba9-3069-91501b4c92fe@redhat.com> Date: Wed, 5 May 2021 18:25:33 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <29db0d1f-2d60-7df6-e6d0-97b89cf636ef@virtuozzo.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.693, 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=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: Kevin Wolf , gregory.farnum@dreamhost.com, qemu-devel@nongnu.org, morita.kazutaka@lab.ntt.co.jp Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 05.05.21 17:05, Vladimir Sementsov-Ogievskiy wrote: > 03.05.2021 12:54, Max Reitz wrote: >> In the bdrv_snapshot_goto() fallback code, we work with a pointer to >> either bs->file or bs->backing. > >> We close that child, > > Do we? We *detach it. >> close the node >> (with .bdrv_close()), apply the snapshot on the child node, and then >> re-open the node (with .bdrv_open()). >> >> In order for .bdrv_open() to attach the same child node that we had >> before, we pass "file={child-node}" or "backing={child-node}" to it. >> Therefore, when .bdrv_open() has returned success, we can assume that >> bs->file or bs->backing (respectively) points to our original child >> again.  This is verified by an assertion. >> >> All of this is not immediately clear from a quick glance at the code, >> so add a comment to the assertion what it is for, and why it is valid. >> It certainly confused Coverity. >> >> Reported-by: Coverity (CID 1452774) >> Signed-off-by: Max Reitz >> --- >>   block/snapshot.c | 14 +++++++++++++- >>   1 file changed, 13 insertions(+), 1 deletion(-) >> >> diff --git a/block/snapshot.c b/block/snapshot.c >> index e8ae9a28c1..cce5e35b35 100644 >> --- a/block/snapshot.c >> +++ b/block/snapshot.c >> @@ -275,13 +275,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs, >>           qobject_unref(file_options); >>           g_free(subqdict_prefix); >> +        /* Force .bdrv_open() below to re-attach fallback_bs on >> *fallback_ptr */ >>           qdict_put_str(options, (*fallback_ptr)->name, >>                         bdrv_get_node_name(fallback_bs)); >> +        /* Now close bs, apply the snapshot on fallback_bs, and >> re-open bs */ >>           if (drv->bdrv_close) { >>               drv->bdrv_close(bs); >>           } >> +        /* .bdrv_open() will re-attach it */ >>           bdrv_unref_child(bs, *fallback_ptr); >>           *fallback_ptr = NULL; >> @@ -296,7 +299,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs, >>               return ret < 0 ? ret : open_ret; >>           } >> -        assert(fallback_bs == (*fallback_ptr)->bs); >> +        /* >> +         * fallback_ptr is &bs->file or &bs->backing.  *fallback_ptr >> +         * was closed above and set to NULL, but the .bdrv_open() call >> +         * has opened it again, because we set the respective option >> +         * (with the qdict_put_str() call above). >> +         * Assert that .bdrv_open() has attached some child on >> +         * *fallback_ptr, and that it has attached the one we wanted >> +         * it to (i.e., fallback_bs). >> +         */ >> +        assert(*fallback_ptr && fallback_bs == (*fallback_ptr)->bs); >>           bdrv_unref(fallback_bs); >>           return ret; >>       } >> > > Reviewed-by: Vladimir Sementsov-Ogievskiy > > === > > I still think that this fallback has more problems.. Nothing guarantee > that all format drivers (even restricted to those who have only one > child) support such logic. > > For example, .bdrv_open() may rely on state structure were zeroed and > not initialize some fields. And .bdrv_close() may just g_free() some > things, not setting them to zero.. So we probably should call > bdrv_open()/bdrv_close() generic functions. But we can't: at least > bdrv_close() requires that node has no parents. > > Not saying about that we lose everything on failure (when actually, it's > better to restore original state on failure). > > This feature should instead be done with help of bdrv_reopen_multiple(), > and even with it it's not obvious how to do it properly. > > The feature were done long ago in 2010 with commit > 7cdb1f6d305e1000b5f882257cbee71b8bb08ef5 by Morita Kazutaka. > > Note also, that the only protocol driver that support snapshots is rbd, > and snapshot support was added to it in 2012 with commit > bd6032470631d8d5de6db84ecb55398b70d9d2f3 by Gregory Farnum. > > Other two drivers with support are sheepdog (which is deprecated) and > qcow2 (I doubt that it should be used as protocol driver for some other > format). > > > Do we really need this fallback? Is it used somewhere? May be, we can > just deprecate it, and look will someone complain? Maybe? :) Max