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Wong" Cc: brauner@kernel.org, cem@kernel.org, dchinner@redhat.com, hch@lst.de, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, ojaswin@linux.ibm.com, ritesh.list@gmail.com, martin.petersen@oracle.com References: <20250204120127.2396727-1-john.g.garry@oracle.com> <20250204120127.2396727-7-john.g.garry@oracle.com> <20250205200517.GZ21808@frogsfrogsfrogs> <58f630a4-3e02-451c-bd6e-22427cec5c11@oracle.com> <20250206214417.GW21808@frogsfrogsfrogs> Content-Language: en-US From: John Garry Organization: Oracle Corporation In-Reply-To: <20250206214417.GW21808@frogsfrogsfrogs> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: AM0PR02CA0168.eurprd02.prod.outlook.com (2603:10a6:20b:28d::35) To DM6PR10MB4313.namprd10.prod.outlook.com (2603:10b6:5:212::20) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR10MB4313:EE_|IA0PR10MB6724:EE_ X-MS-Office365-Filtering-Correlation-Id: fe7165d2-2fda-41dc-b9f6-08dd476d481b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024; 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Wong wrote: >>> What is this checking? That something else already created a mapping in >>> the COW fork, so we want to bail out to get rid of it? >> I want to check if some data is shared. In that case, we should unshare. > Why is it necessary to unshare? Userspace gave us a buffer of new > contents, and we're already prepared to write that out of place and > remap it. fine, as long as the remap does what we need, then I won't bother with this explicit unshare. > >> And I am not sure if that check is sufficient. >> >> On the buffered write path, we may have something in a CoW fork - in that >> case it should be flushed, right? > Flushed against what? Concurrent writeback or something? The directio > setup should have flushed dirty pagecache, so the only things left in > the COW fork are speculative preallocations. (IOWs, I don't understand > what needs to be flushed or why.) ah, ok, as long as DIO would have flushed dirty relevant pagecache, then we should be good. Cheers, John