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Thu, 20 Aug 2020 08:31:55 +0000 (UTC) Subject: Re: [PATCH v7 14/47] stream: Deal with filters To: Kevin Wolf References: <20200625152215.941773-1-mreitz@redhat.com> <20200625152215.941773-15-mreitz@redhat.com> <20200818142851.GD6865@linux.fritz.box> <59fe8d64-b073-aeff-1afb-b526e9a085f1@redhat.com> <20200819151625.GF10272@linux.fritz.box> From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: Date: Thu, 20 Aug 2020 10:31:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200819151625.GF10272@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0.0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="T0Iaj9S30duijnZRkOdDzPfEnOEi0bvRm" Received-SPF: pass client-ip=205.139.110.120; envelope-from=mreitz@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/20 03:03:32 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --T0Iaj9S30duijnZRkOdDzPfEnOEi0bvRm Content-Type: multipart/mixed; boundary="dJvAORsBh9vnEHyO4DwIDHjPBFoLf2dKh" --dJvAORsBh9vnEHyO4DwIDHjPBFoLf2dKh Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 19.08.20 17:16, Kevin Wolf wrote: > Am 19.08.2020 um 16:47 hat Max Reitz geschrieben: >> On 18.08.20 16:28, Kevin Wolf wrote: >>> Am 25.06.2020 um 17:21 hat Max Reitz geschrieben: >>>> Because of the (not so recent anymore) changes that make the stream jo= b >>>> independent of the base node and instead track the node above it, we >>>> have to split that "bottom" node into two cases: The bottom COW node, >>>> and the node directly above the base node (which may be an R/W filter >>>> or the bottom COW node). >>>> >>>> Signed-off-by: Max Reitz >>>> --- >>>> qapi/block-core.json | 4 +++ >>>> block/stream.c | 63 ++++++++++++++++++++++++++++++++-----------= - >>>> blockdev.c | 4 ++- >>>> 3 files changed, 53 insertions(+), 18 deletions(-) >>>> >>>> diff --git a/qapi/block-core.json b/qapi/block-core.json >>>> index b20332e592..df87855429 100644 >>>> --- a/qapi/block-core.json >>>> +++ b/qapi/block-core.json >>>> @@ -2486,6 +2486,10 @@ >>>> # On successful completion the image file is updated to drop the back= ing file >>>> # and the BLOCK_JOB_COMPLETED event is emitted. >>>> # >>>> +# In case @device is a filter node, block-stream modifies the first n= on-filter >>>> +# overlay node below it to point to base's backing node (or NULL if @= base was >>>> +# not specified) instead of modifying @device itself. >>> >>> Not to @base's backing node, but to @base itself (or actually, to >>> above_base's backing node, which is initially @base, but may have >>> changed when the job is completed). >> >> Oh, yes. >> >> (I thought I had noticed that already at some point and fixed it >> locally... But apparently not.) >> >>> Should we also document what using a filter node for @base means? >> >> Hm. What does it mean? I think the more interesting case is what it >> means if above_base is a filter, right? >> >> Maybe we can put in somewhere in the =E2=80=9CIf a base file is specifie= d then >> sectors are not copied from that base file and its backing chain.=E2=80= =9D But >> the more I think about it, the less I know what we could add to it. >> What happens if there are filters above @base is that their data isn=E2= =80=99t >> copied, because that=E2=80=99s exactly the data in @base. >=20 > The interesting part is probably the graph reconfiguration at the end of > the job. Which is actually already documented: >=20 > # When streaming completes the image file will have the base > # file as its backing file. >=20 > Of course, this is not entirely correct any more (because the base may > have changed). >=20 > If @base is a filter, what backing file path do we write into the top > layer? A json: filename including the filter? Yes. Or, actually. Now that I read the code... It takes @base=E2=80=99s filena= me before the stream job and then uses that. So if @base has changed during the job, then it still uses the old filename. But that=E2=80=99s not really due to this series. > Is this worth mentioning > or do you consider it obvious? Hm. I consider it obvious, yes. @base becomes @top=E2=80=99s backing file= (at least without any graph changes while the job is running), so naturally what=E2=80=99s written into the image header is @base=E2=80=99s filename = =E2=80=93 which is a json:{} filename. On second thought, @backing-file=E2=80=99s description mysteriously states = that =E2=80=9CQEMU will automatically determine the backing file string to use= =E2=80=9D. Which makes sense because it would clearly not make sense to describe what=E2=80=99s actually happening, which is to use @base=E2=80=99s filename= at job start regardless of whether it=E2=80=99s still there at the end of the job. So I suppose I have the choice of either documenting exactly what=E2=80=99s happening, even though it doesn=E2=80=99t make much sense, or just not, kee= ping it mysterious. So all in all, I believe the biggest surprise about what=E2=80=99s written = into the top layer isn=E2=80=99t that it may be a json:{} filename, but the file= name of a node that maybe doesn=E2=80=99t even exist anymore? (Oh, no, please d= on=E2=80=99t tell me you can delete it and get an invalid pointer read...) The more I think about it, the more I think there are problems beyond the scope of this series here. :/ Max --dJvAORsBh9vnEHyO4DwIDHjPBFoLf2dKh-- --T0Iaj9S30duijnZRkOdDzPfEnOEi0bvRm Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl8+NPkACgkQ9AfbAGHV z0CzTgf9HZeKzqeJEBs2pLMx6jI9aR+ivWMIMrSYV2EXH2WQD2TWsF6Hn73MSTIn mGy2JWj+DRmolhOH2ytIOtSTWeI0fMBlRJRR9xQY6/PnUm8oaVbMcOLpEM17F4kr PUxz4xqd7lRL0abcnJosbFgb+98K9adTczDWi9t4vsm9boIUgGRZennUav3VfwHk AsSHXnvk+f1Mb5W6SWyY3N4VgpPHxpcj0C5h9gj8/UuM3NNy2P500D3jNuIzq4g2 6scKRCbm7hodMOws3i2gl97pdjCBwst3jjMfvrJSQ8ljI3v5ekJdih/z/NR/mw+I i94RQZMpwjFMzEOigZ6vGBfcYgmVQw== =FLIV -----END PGP SIGNATURE----- --T0Iaj9S30duijnZRkOdDzPfEnOEi0bvRm--