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Wed, 6 Nov 2024 19:59:47 +0000 Date: Wed, 6 Nov 2024 11:59:44 -0800 From: Dan Williams To: Jan Kara , Asahi Lina CC: Dave Chinner , Jan Kara , Dan Williams , Matthew Wilcox , "Alexander Viro" , Christian Brauner , Sergio Lopez Pascual , , , , Subject: Re: [PATCH] dax: Allow block size > PAGE_SIZE Message-ID: <672bcab0911a2_10bc62943f@dwillia2-xfh.jf.intel.com.notmuch> References: <20241101-dax-page-size-v1-1-eedbd0c6b08f@asahilina.net> <20241104105711.mqk4of6frmsllarn@quack3> <7f0c0a15-8847-4266-974e-c3567df1c25a@asahilina.net> <21f921b3-6601-4fc4-873f-7ef8358113bb@asahilina.net> <20241106121255.yfvlzcomf7yvrvm7@quack3> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20241106121255.yfvlzcomf7yvrvm7@quack3> X-ClientProxiedBy: MW2PR16CA0057.namprd16.prod.outlook.com (2603:10b6:907:1::34) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) 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: PH8PR11MB8107:EE_|DS0PR11MB7998:EE_ X-MS-Office365-Filtering-Correlation-Id: 6ba8c160-cdcf-4bee-8556-08dcfe9d90c9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|1800799024|366016; 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Right now it is the > > only thing in DAX code complaining on a page size/block size mismatch > > (at least for virtiofs). If this is so important, I feel like there > > should be a higher level check elsewhere, like something happening at > > mount time or on file open. It should actually cause the operations to > > fail cleanly. > > That's a fair point. Currently filesystems supporting DAX check for this in > their mount code because there isn't really a DAX code that would get > called during mount and would have enough information to perform the check. > I'm not sure adding a new call just for this check makes a lot of sense. > But if you have some good place in mind, please tell me. Is not the reason that dax_writeback_mapping_range() the only thing checking ->i_blkbits because 'struct writeback_control' does writeback in terms of page-index ranges? All other dax entry points are filesystem controlled that know the block-to-pfn-to-mapping relationship. Recall that dax_writeback_mapping_range() is historically for pmem persistence guarantees to make sure that applications write through CPU cache to media. Presumably there are no cache coherency concerns with fuse and dax writes from the guest side are not a risk of being stranded in CPU cache. Host side filesystem writeback will take care of them when / if the guest triggers a storage device cache flush, not a guest page cache writeback. So, I think the simplest fix here is potentially: diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c index 12ef91d170bb..15cf7bb20b5e 100644 --- a/fs/fuse/dax.c +++ b/fs/fuse/dax.c @@ -777,11 +777,8 @@ ssize_t fuse_dax_write_iter(struct kiocb *iocb, struct iov_iter *from) static int fuse_dax_writepages(struct address_space *mapping, struct writeback_control *wbc) { - - struct inode *inode = mapping->host; - struct fuse_conn *fc = get_fuse_conn(inode); - - return dax_writeback_mapping_range(mapping, fc->dax->dev, wbc); + /* nothing to flush, fuse cache coherency is managed by the host */ + return 0; } static vm_fault_t __fuse_dax_fault(struct vm_fault *vmf, unsigned int order,