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Tue, 11 Jan 2022 05:27:59 +0000 Message-ID: Date: Tue, 11 Jan 2022 13:27:49 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.4.1 Subject: Re: [PATCH v4 3/4] btrfs: add device major-minor info in the struct btrfs_device Content-Language: en-US To: kreijack@inwind.it, linux-btrfs@vger.kernel.org Cc: josef@toxicpanda.com, dsterba@suse.com, nborisov@suse.com, l@damenly.su References: <9dca55580c33938776b9024cc116f9f6913a65cf.1641794058.git.anand.jain@oracle.com> <03c7c3d2-5abe-0087-90d9-698c77a98fc4@libero.it> From: Anand Jain In-Reply-To: <03c7c3d2-5abe-0087-90d9-698c77a98fc4@libero.it> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SG2PR02CA0051.apcprd02.prod.outlook.com (2603:1096:4:54::15) To MN2PR10MB4128.namprd10.prod.outlook.com (2603:10b6:208:1d2::24) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 5bc086a9-e8e1-4ab4-56ef-08d9d4c3215c X-MS-TrafficTypeDiagnostic: MN2PR10MB3789:EE_ X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:8882; 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>> +    /* Device's major-minor number */ >> +    dev_t devt; > > What is the relation between > >     device->devt > > and > >     device->bdev->bd_dev > > ? Both are same. device->bdev gets an update at the time device open. Otherwise, it is null. > I assumed that there are situation where there is no a device connected > to a btrfs_device > structure (e.g. a degraded mounted filesystem where a device is > missing); in this case > does devt == 0 ? Even for the missing device we do call add_missing_dev() -> btrfs_alloc_device() that will ensure devt == 0. > But are there cases where devt != 0 (a device is associated to a block > device structure) and bdev == NULL ? It is possible- When we unmount, the btrfs_device continues to exist and, both device->name and device->devt shall not be NULL/0; However, device->bdev will be NULL; If the device is scanned again with a different uuid, then the free_stale_device() is called to check and free the old/stale struct btrfs_device. Thanks, Anand