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Wed, 29 Apr 2020 07:39:41 +0000 (UTC) Subject: Re: [PATCH 1/4] block: Add bdrv_make_empty() To: Kevin Wolf References: <20200428132629.796753-1-mreitz@redhat.com> <20200428132629.796753-2-mreitz@redhat.com> <20200428142104.GI5789@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: Wed, 29 Apr 2020 09:39:39 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200428142104.GI5789@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="JELVLaOGRUFLtAwhRmeBpjGTYFhvsltma" Received-SPF: pass client-ip=205.139.110.61; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/29 00:53:13 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 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: 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) --JELVLaOGRUFLtAwhRmeBpjGTYFhvsltma Content-Type: multipart/mixed; boundary="KuoTs7hNC8rCu5jabmJajhuoq1Qemz58b" --KuoTs7hNC8rCu5jabmJajhuoq1Qemz58b Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 28.04.20 16:21, Kevin Wolf wrote: > Am 28.04.2020 um 15:26 hat Max Reitz geschrieben: >> Right now, all users of bdrv_make_empty() call the BlockDriver method >> directly. That is not only bad style, it is also wrong, unless the >> caller has a BdrvChild with a WRITE permission. >> >> Introduce bdrv_make_empty() that verifies that it does. >> >> Signed-off-by: Max Reitz >> --- >> include/block/block.h | 1 + >> block.c | 23 +++++++++++++++++++++++ >> 2 files changed, 24 insertions(+) >> >> diff --git a/include/block/block.h b/include/block/block.h >> index b05995fe9c..d947fb4080 100644 >> --- a/include/block/block.h >> +++ b/include/block/block.h >> @@ -351,6 +351,7 @@ BlockMeasureInfo *bdrv_measure(BlockDriver *drv, Qem= uOpts *opts, >> void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr); >> void bdrv_refresh_limits(BlockDriverState *bs, Error **errp); >> int bdrv_commit(BlockDriverState *bs); >> +int bdrv_make_empty(BdrvChild *c, Error **errp); >> int bdrv_change_backing_file(BlockDriverState *bs, >> const char *backing_file, const char *backing_fmt); >> void bdrv_register(BlockDriver *bdrv); >> diff --git a/block.c b/block.c >> index 2e3905c99e..b0d5b98617 100644 >> --- a/block.c >> +++ b/block.c >> @@ -6791,3 +6791,26 @@ void bdrv_del_child(BlockDriverState *parent_bs, = BdrvChild *child, Error **errp) >> =20 >> parent_bs->drv->bdrv_del_child(parent_bs, child, errp); >> } >> + >> +int bdrv_make_empty(BdrvChild *c, Error **errp) >> +{ >> + BlockDriver *drv =3D c->bs->drv; >> + int ret; >> + >> + assert(c->perm & BLK_PERM_WRITE); >=20 > If I understand correctly, bdrv_make_empty() is called to drop an > overlay whose content is identical to what it would read from its > backing file (in particular after a commit operation). This means that > the caller promises that the visible content doesn't change. >=20 > So should we check BLK_PERM_WRITE_UNCHANGED instead? Ah, right. Yes, that would be better. (Or to check both, whether any of them has been taken.) Max --KuoTs7hNC8rCu5jabmJajhuoq1Qemz58b-- --JELVLaOGRUFLtAwhRmeBpjGTYFhvsltma Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl6pLzsACgkQ9AfbAGHV z0D/bAf7BceJHRlYifeCSsI/PoUyGXfv5/gXVSIf4TjHaef5dnxNtxrBz56ZjOOc 0pMGOQkAbfHn9CKejuZaQYIJG9OxbH8EuIPO1gq6yhzqQSNsxu1Q9H6WgVLTZZ8b drxjaAoo2YtxxN1BifzIL7O7mz7uLBgpaN6lxXXw3+ANvlYt5b3zqIGbeodKS7cG 0GkOjJBKtA02HPbUmtLdbeKxhEtbdNIBE/3DS/fad02qoQA8wxjkRiMdv48R+TrE VW5IuAqwV9tJ2lzdaacgB8OlH5w8V1l2tWFUY6Ro3p0WvO4gb3XxaTvOGcSVu2cT rVORXdvNNdDpQjKgq9Yft3hxrP2VWw== =G1I7 -----END PGP SIGNATURE----- --JELVLaOGRUFLtAwhRmeBpjGTYFhvsltma--