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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, tytso@mit.edu, linux-ext4@vger.kernel.org References: <20250213135619.1148432-1-john.g.garry@oracle.com> <20250213135619.1148432-5-john.g.garry@oracle.com> <20250224195912.GC21808@frogsfrogsfrogs> Content-Language: en-US From: John Garry Organization: Oracle Corporation In-Reply-To: <20250224195912.GC21808@frogsfrogsfrogs> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO4P123CA0013.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:150::18) 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_|PH7PR10MB6201:EE_ X-MS-Office365-Filtering-Correlation-Id: dec43bcd-1432-4d9e-48c3-08dd5585f203 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|1800799024|376014; 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Wong wrote: >> + * ``IOMAP_ATOMIC_COW``: This write is being issued with torn-write >> + protection based on CoW support. > I think using "COW" here results in a misnamed flag. Consider: > > "IOMAP_ATOMIC_SW: ok, fine > This write is being issued with torn-write protection > via a software fallback provided by the filesystem." I'm not sure that we really need to even mention software fallback. Indeed, xfs could just use IOMAP_ATOMIC_SW always when the bdev does not support HW offload. Maybe I can mention that typically it can be used as a software fallback when HW offload is not possible. > > iomap itself doesn't care*how* the filesystem guarantees that the > direct write isn't torn, right? Correct. iomap just ensures that for IOMAP_ATOMIC_HW we produce a single bio - that's the only check really. > The fs' io completion handler has to > ensure that the mapping update(s) are either applied fully or discarded > fully. right > > In theory if you had a bunch of physical space mapped to the same > file but with different unwritten states, you could gang together all > the unwritten extent conversions in a single transaction, which would > provide the necessary tearing prevention without the out of place write. > Nobody does that right now, but I think that's the only option for ext4. ok, maybe. But ext4 still does have bigalloc or opportunity to support forcealign (to always use IOMAP_ATOMIC_HW for large untorn writes). Thanks, John