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Mon, 9 Sep 2019 07:56:29 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-116-179.ams2.redhat.com [10.36.116.179]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 80CE55D9D6; Mon, 9 Sep 2019 07:56:28 +0000 (UTC) To: Kevin Wolf References: <20190809161407.11920-1-mreitz@redhat.com> <20190809161407.11920-5-mreitz@redhat.com> <20190904161658.GD21246@localhost.localdomain> From: Max Reitz Openpgp: preference=signencrypt 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: Mon, 9 Sep 2019 09:56:26 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190904161658.GD21246@localhost.localdomain> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="sE7iPZw0kF0n1SCCRXym4PeU3sht7VFzF" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.68]); Mon, 09 Sep 2019 07:56:30 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH v6 04/42] block: Add child access functions 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) --sE7iPZw0kF0n1SCCRXym4PeU3sht7VFzF Content-Type: multipart/mixed; boundary="gBVW5bPxu6W6C05cuN1Y9FI54tCjAX18c"; protected-headers="v1" From: Max Reitz To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Vladimir Sementsov-Ogievskiy Message-ID: Subject: Re: [PATCH v6 04/42] block: Add child access functions References: <20190809161407.11920-1-mreitz@redhat.com> <20190809161407.11920-5-mreitz@redhat.com> <20190904161658.GD21246@localhost.localdomain> In-Reply-To: <20190904161658.GD21246@localhost.localdomain> --gBVW5bPxu6W6C05cuN1Y9FI54tCjAX18c Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 04.09.19 18:16, Kevin Wolf wrote: > Am 09.08.2019 um 18:13 hat Max Reitz geschrieben: >> There are BDS children that the general block layer code can access, >> namely bs->file and bs->backing. Since the introduction of filters an= d >> external data files, their meaning is not quite clear. bs->backing ca= n >> be a COW source, or it can be an R/W-filtered child; bs->file can be a= n >> R/W-filtered child, it can be data and metadata storage, or it can be >> just metadata storage. >> >> This overloading really is not helpful. This patch adds function that= >> retrieve the correct child for each exact purpose. Later patches in >> this series will make use of them. Doing so will allow us to handle >> filter nodes and external data files in a meaningful way. >> >> Signed-off-by: Max Reitz >> Reviewed-by: Vladimir Sementsov-Ogievskiy >=20 > Each time I look at this patch, I'm confused by the function names. > Maybe I should just ask what the idea there was, or more specifically: > What does the "filtered" in "filtered child" really mean? >=20 > Apparently any child of a filter node is "filtered" (which makes sense)= , It isn=E2=80=99t, filters can have non-filter children. For example, bac= kup-top could have the source as a filtered child and the target as a non-filter child. > but also bs->backing of a qcow2 image, while bs->file of qcow2 isn't. > raw doesn't have any "filtered" child. What's the system behind this? =E2=80=9Cfiltered=E2=80=9D means: If the parent node returns data from th= is child, it won=E2=80=99t modify it, neither its content nor its position. COW and R= /W filters differ in how they handle writes; R/W filters pass them through to the filtered child, COW filters copy them off to some other child node (and then the filtered child=E2=80=99s data will no longer be visibl= e at that location). The main reason behind the common =E2=80=9Cfiltered=E2=80=9D name is for = the generic functions that work on both COW and true filter (R/W filters) chains. We need such functionality sometimes. I personally felt like the concept of true (R/W) filters and COW children was similar enough to share a common name base. qcow2 has a COW child. As such, it acts as a COW filter in the sense of the function names. raw has neither a COW child nor acts as an R/W filter. As such, it has no filtered child. My opinion on this hasn=E2=80=99t changed. (To reiterate, in practice I see no way anyone would ever use raw as an R/W filter. Either you use it without offset/size, in which case you simply use it in lieu of a format node, so you precisely don=E2=80=99t want it to act a= s a filter when it comes to allocation information and so on (even though it can be classified a filter here). Or you use it as kind of a filter with offset/size, but then it no longer is a filter. Filters are defined by =E2=80=9CEvery filter must fulfill these condition= s: ...=E2=80=9D =E2=80=93 not by =E2=80=9CEverything that fulfills these conditions is a = filter=E2=80=9D. Marking a driver as a filter has consequences, and I don=E2=80=99t see wh= y we would want those consequences for raw.) > It looks like bdrv_filtered_child() is the right function to iterate > along a backing file chain, but I just still fail to connect that and > the name of the function in a meaningful way. It=E2=80=98s the right function to iterate along a filter chain. This in= cludes COW backing children and R/W filtered children. >> +/* >> + * Return the child that @bs acts as an overlay for, and from which d= ata may be >> + * copied in COW or COR operations. Usually this is the backing file= =2E >> + */ >=20 > Or NULL, if no such child exists. >=20 > It's relatively obvious here, but for some of the functions further dow= n > it would be really good to describe in which cases NULL is expected (or= > that NULL is even a possible return value). I=E2=80=99ll look into it. Max --gBVW5bPxu6W6C05cuN1Y9FI54tCjAX18c-- --sE7iPZw0kF0n1SCCRXym4PeU3sht7VFzF Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl12BaoACgkQ9AfbAGHV z0AF1wgAguyaf2SC8PxeMpzE5YTKXKTpJe6/H5fw/3fWR+m7iPedOhW1nraNMtPe KSel2exgatuZ8A5FCqTqGBUBsc0riTYyYgDhzQL4OluUr6L5ziAmuTySyEFGU7Su ec0EHLHxkKDjjVPKzU9Vxg7e+zazCmiiHpQLcLMq3avMWH6CQ1Pr7p9JLqQvMt2p E7dH53hlKPZrwzJ/wbCveMVDfL4iSrF1OL2ub+TuZuuAC/sMDSM4ZN+dTIXyvUGS VWDaT5/cjht2MQYVfRRXvRsKmSdMxO7E4+VLwRQFu4D7BcwIwpL8d4YnI4xFw4Bd 9Xteh2cYCFwskTWV+F0CzxpoJ3JhYQ== =kH1W -----END PGP SIGNATURE----- --sE7iPZw0kF0n1SCCRXym4PeU3sht7VFzF--