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dkim=none (message not signed) header.d=none;redhat.com; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AM6PR08MB5077.eurprd08.prod.outlook.com (2603:10a6:20b:e6::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4108.27; Fri, 7 May 2021 07:11:49 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133%7]) with mapi id 15.20.4108.029; Fri, 7 May 2021 07:11:49 +0000 Subject: Re: [PATCH v4 2/6] block: Allow changing bs->file on reopen To: Alberto Garcia , qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, Kevin Wolf , Max Reitz References: <31ccb1061199ee11bf9879f6c60608a19b83263d.1616000692.git.berto@igalia.com> From: Vladimir Sementsov-Ogievskiy Message-ID: Date: Fri, 7 May 2021 10:11:47 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 In-Reply-To: <31ccb1061199ee11bf9879f6c60608a19b83263d.1616000692.git.berto@igalia.com> Content-Type: text/plain; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR03-DB5-obe.outbound.protection.outlook.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 17.03.2021 20:15, Alberto Garcia wrote: > When the x-blockdev-reopen was added it allowed reconfiguring the > graph by replacing backing files, but changing the 'file' option was > forbidden. Because of this restriction some operations are not > possible, notably inserting and removing block filters. I now started to work on making backup-top filter public.. And I think, we'll need separate commands to insert/remove filters anyway.. As blockdev-reopen has the following problems: 1. It can't append filter above top node, connected to block-device. (but bdrv_append() can do it) 2. It can't simultaneously create new node and append it. This is important for backup-top filter, which unshares write even when has no writing parent. Now bdrv_append() works in a smart way for it: it first do both graph modification (add child to filter, and replace original node by filter) and then update graph permissions. So, we'll need a command which in one roll create filter node and inserts it where needed. 3. blockdev-reopen requires to specify all options (otherwise, they will be changed to default). So it requires passing a lot of information. But we don't need to touch any option of original bs parent to insert a filter between parent and bs. In other words, we don't want to reopen something. We want to insert filter. === Hmm, another mentioned use of blockdev-reopen was reopening some RO node to RW, to modify bitmaps.. And here again, blockdev-reopen is not very convenient: 1. Again, it requires to specify all options (at least, that was not default on node open).. And this only to change one property: read-only. Seems overcomplicated. 2. Bitmaps modifications are usually done in transactions. Adding a clean transaction support for small command that reopens only to RW, and back to RO on transaction finalization seems simpler, than for entire generic blockdev-reopen. === Hmm, interesting. x-blockdev-reopen says that not specified options are reset to default. x-blockdev-reopen works through bdrv_reopen_multiple, so I think bdrv_reopen_mutliple should reset options to default as well. Now look at bdrv_reopen_set_read_only(): it specifies only one option: "read-only". This means that all other options will be reset to default. But for sure, callers of bdrv_reopen_set_read_only() want only to change read-only status of node, not all other options. Do we have a bug here? -- Best regards, Vladimir