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Thu, 16 Jul 2020 14:50:55 +0000 (UTC) Subject: Re: [PATCH v7 02/47] block: Add chain helper functions To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20200625152215.941773-1-mreitz@redhat.com> <20200625152215.941773-3-mreitz@redhat.com> <697fa19f-b68f-f45d-7fab-a0d746c44557@virtuozzo.com> 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, 16 Jul 2020 16:50:50 +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: <697fa19f-b68f-f45d-7fab-a0d746c44557@virtuozzo.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="mLzvLipRwVmryBgtVxlM4zJH4VvgPoDIO" Received-SPF: pass client-ip=207.211.31.120; envelope-from=mreitz@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/16 04:25:38 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 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_H2=-1, 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 , qemu-devel@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) --mLzvLipRwVmryBgtVxlM4zJH4VvgPoDIO Content-Type: multipart/mixed; boundary="epJ82nuw8UnAxgR9QopP0hVXRQi4wJ01l" --epJ82nuw8UnAxgR9QopP0hVXRQi4wJ01l Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 13.07.20 12:18, Vladimir Sementsov-Ogievskiy wrote: > 25.06.2020 18:21, Max Reitz wrote: >> Add some helper functions for skipping filters in a chain of block >> nodes. >> >> Signed-off-by: Max Reitz >> --- >> =C2=A0 include/block/block_int.h |=C2=A0 3 +++ >> =C2=A0 block.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | 55 ++++++++++++++++++= +++++++++++++++++++++ >> =C2=A0 2 files changed, 58 insertions(+) >> >> diff --git a/include/block/block_int.h b/include/block/block_int.h >> index bb3457c5e8..5da793bfc3 100644 >=20 >=20 > This patch raises two questions: >=20 > 1. How to treat filters at the end of the backing chain? It was my understanding that this configuration would be impossible. > - child-access function will return no filter child for such nodes, it's > correct of course > - filer skipping functions will return this filter.. How much is it > correct - I don't know. >=20 >=20 > Consider a chain >=20 > top --- backing ---> filter-with-no-child How would it be possible to have filter without a child? > if bdrv_backing_chain_next(top) returns NULL, it's incorrect, because > top actually have backing, and on read it will read from it for > unallocated clusters (and this should crash). So, probably, returning > filter as a backing-chain-next is a valid thing to do. Or we should > assert that we are not in such situation (which may crash more often > than trying to really read from nonexistent child). >=20 > so, returning NULL, may even less correct than returning a filter.. >=20 >=20 > 2. How to tread nodes with drv=3DNULL, but with filter child (with > BDRV_CHILD_FILTERED role). drv=3DNULL is a bug on its own that should be fixed... (The idea we had at some point was to introduce a special driver that just always returns -EIO on everything, and to replace corrupt qcow2 nodes by that. Or, well, we might just return -EIO from the qcow2 driver, actually, if we never use drv=3DNULL anywhere else.) In any case, drv=3DNULL is an edge case that I think never has anything to do with filters. > - child-access functions returns no filtered child for such nodes > - filter skipping functions will stop on it.. >=20 > =3D=3D=3D=3D=3D=3D=3D >=20 > Isn't it better to drop drv->is_filter at all? And call filter nodes > with a bs->file or bs->backing > child in BDRV_CHILD_FILTERED role? This automatically closes the two > questions: >=20 > - node without a child in BDRV_CHILD_FILTERED is automatically > non-filter. So, filter driver is responsible for having such child. > - node without a drv may still be a filter if it have > BDRV_CHILD_FILTERED.. Still, not very useful. >=20 > Anyway, is_filter and BDRV_CHILD_FILTERED are in contradiction, and it > seems good to get rid of is_filter. But I may miss something. >=20 > [..] >=20 >> + >> +static BlockDriverState *bdrv_do_skip_filters(BlockDriverState *bs, >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bool >> stop_on_explicit_filter) >> +{ >> +=C2=A0=C2=A0=C2=A0 BdrvChild *c; >> + >> +=C2=A0=C2=A0=C2=A0 if (!bs) { >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return NULL; >> +=C2=A0=C2=A0=C2=A0 } >> + >> +=C2=A0=C2=A0=C2=A0 while (!(stop_on_explicit_filter && !bs->implicit)) = { >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 c =3D bdrv_filter_child(bs); >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (!c) { >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 brea= k; >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bs =3D c->bs; >> +=C2=A0=C2=A0=C2=A0 } >> +=C2=A0=C2=A0=C2=A0 /* >> +=C2=A0=C2=A0=C2=A0=C2=A0 * Note that this treats nodes with bs->drv =3D= =3D NULL as not being >> +=C2=A0=C2=A0=C2=A0=C2=A0 * filters (bs->drv =3D=3D NULL should be repla= ced by something else >> +=C2=A0=C2=A0=C2=A0=C2=A0 * anyway). >> +=C2=A0=C2=A0=C2=A0=C2=A0 * The advantage of this behavior is that this = function will thus >> +=C2=A0=C2=A0=C2=A0=C2=A0 * always return a non-NULL value (given a non-= NULL @bs). >=20 > I don't see, how it is follows from first sentence? We can skip nodes > with a child of BDRV_CHILD_FILTERED and drv=3DNULL as well, and still ret= urn > non-NULL bs at the end... My idea was that nodes with bs->drv =3D=3D NULL might not even have children. If we treated them like filters and skipped through them, we would have to return NULL if there is no child. > Didn't you mean "treat nodes without filter child as not being filters, > even if they have drv->is_filter =3D=3D true"? This is a real reason for = the > second sentence. Hm. I implicitly always assume that filters always have a filter child, so I tend to not even question that part. Max --epJ82nuw8UnAxgR9QopP0hVXRQi4wJ01l-- --mLzvLipRwVmryBgtVxlM4zJH4VvgPoDIO Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl8QaUsACgkQ9AfbAGHV z0CUmAf/bhBFQjns4pIZJfM3zuXE+gRP5DEGhKKjixi2EEPvd4Bb1GLWnoJgbSyK 5ibwewZC7YOdll9xbu/tjIe54qTnTN9FveBaokQT4k/ARVaNqNgfSTi9Ioes9SO/ 2xN+tzvv0kMLM55j3gvBxQz9bBfdGlRpxun+nwHiAqSC1haGuck3T8mHQuGxF2W+ 4T7NXHOc2jhTmH/OgHhvIkZ76F6Ka6kbPX3q6GRp4XBkqkMajJNg+VUB4us1I39a 9Y5PMUt7h7s/7tK39XjgJE5VySBHV/W85DCD9G0pUIYc8iyplvYzM62U1jcYXS1H g43HnhIxhNuiPjKyB8qSSKtUhJe4Mg== =IJWW -----END PGP SIGNATURE----- --mLzvLipRwVmryBgtVxlM4zJH4VvgPoDIO--