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Piccoli" , linux-btrfs@vger.kernel.org, clm@fb.com, josef@toxicpanda.com, dsterba@suse.com, linux-fsdevel@vger.kernel.org, kernel@gpiccoli.net, kernel-dev@igalia.com, vivek@collabora.com, ludovico.denittis@collabora.com, johns@valvesoftware.com, nborisov@suse.com References: <20230504170708.787361-1-gpiccoli@igalia.com> <20230504170708.787361-2-gpiccoli@igalia.com> <2892ff0d-9225-07b7-03e4-a3c96d0bff59@gmx.com> <20230505133810.GO6373@twin.jikos.cz> <9839c86a-10e9-9c3c-0ddb-fc8011717221@oracle.com> <7eaf251e-2369-1a07-a81f-87e4da8b6780@gmx.com> <20230511115150.GX32559@suse.cz> Content-Language: en-US From: Anand Jain In-Reply-To: <20230511115150.GX32559@suse.cz> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SG2PR02CA0005.apcprd02.prod.outlook.com (2603:1096:3:17::17) To PH0PR10MB5706.namprd10.prod.outlook.com (2603:10b6:510:148::10) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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engine=ICAP:2.0.254,Aquarius:18.0.942,Hydra:6.0.573,FMLib:17.11.170.22 definitions=2023-05-11_11,2023-05-05_01,2023-02-09_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 phishscore=0 adultscore=0 spamscore=0 suspectscore=0 malwarescore=0 bulkscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2304280000 definitions=main-2305110123 X-Proofpoint-GUID: CNti9T7QL6NmgyZu6kDxGuNRoNRXXpxb X-Proofpoint-ORIG-GUID: CNti9T7QL6NmgyZu6kDxGuNRoNRXXpxb Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 11/5/23 19:51, David Sterba wrote: > On Mon, May 08, 2023 at 07:50:55PM +0800, Qu Wenruo wrote: >> On 2023/5/8 19:27, Anand Jain wrote: >>> On 05/05/2023 21:38, David Sterba wrote: >>>> On Fri, May 05, 2023 at 03:21:35PM +0800, Qu Wenruo wrote: >>>>> On 2023/5/5 01:07, Guilherme G. Piccoli wrote: >>>> This is actually a good point, we can do that already. As a conterpart >>>> to 5f58d783fd7823 ("btrfs: free device in btrfs_close_devices for a >>>> single device filesystem") that drops single device from the list, >>>> single fs devices wouldn't be added to the list but some checks could be >>>> still done like superblock validation for eventual error reporting. >>> >>> Something similar occurred to me earlier. However, even for a single >>> device, we need to perform the scan because there may be an unfinished >>> replace target from a previous reboot, or a sprout Btrfs filesystem may >>> have a single seed device. If we were to make an exception for replace >>> targets and seed devices, it would only complicate the scan logic, which >>> goes against our attempt to simplify it. >> >> If we go SINGLE_DEV compat_ro flags, then no such problem at all, we can >> easily reject any multi-dev features from such SINGLE_DEV fs. > For a single device, if we remove device replacement and seeding for a specific flag, scanning is unnecessary. > With the scanning complications that Anand mentions the compat_ro flag > might make more sense, with all the limitations but allowing a safe use > of the duplicated UUIDs. > > The flag would have to be set at mkfs time or by btrfsune on an > unmounted filesystem. Doing that on a mounted filesystem is possible too > but brings problems with updating the state of scanned device, > potentially ongoing operations like dev-replace and more. Setting the flag at mkfs time is preferable, in my opinion. We could still support the btrfstune method and/or online method later. While we are here, I'm taking the opportunity to consolidate the scattered metadata UUID checking, which has been a long-standing goal of mine to clean up. This will make adding multi-UUID support cleaner. If you have any ideas, please share. Thanks, Anand