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header.from=ddn.com; Received: from CH2PR19MB3864.namprd19.prod.outlook.com (2603:10b6:610:93::21) by CH4PR19MB8611.namprd19.prod.outlook.com (2603:10b6:610:22d::10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9253.12; Tue, 21 Oct 2025 22:08:16 +0000 Received: from CH2PR19MB3864.namprd19.prod.outlook.com ([fe80::abe1:8b29:6aaa:8f03]) by CH2PR19MB3864.namprd19.prod.outlook.com ([fe80::abe1:8b29:6aaa:8f03%5]) with mapi id 15.20.9253.011; Tue, 21 Oct 2025 22:08:16 +0000 Message-ID: <2460d0d7-486f-4520-b691-eb189912fade@ddn.com> Date: Wed, 22 Oct 2025 00:08:12 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] fuse: check if system-wide io_uring is enabled To: Jens Axboe , Miklos Szeredi Cc: linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, Pavel Begunkov , Joanne Koong , Luis Henriques References: <20251021-io-uring-fix-check-systemwide-io-uring-enable-v1-1-01d4b4a8ef4f@ddn.com> From: Bernd Schubert 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KILL_LEVEL=7.0 tests=BSF_BESS_OUTBOUND X-BESS-BRTS-Status:1 On 10/21/25 23:56, Jens Axboe wrote: > On 10/21/25 2:31 PM, Bernd Schubert wrote: >> Add check_system_io_uring() to determine if system-wide io_uring is >> available for a FUSE mount. This is useful because FUSE io_uring >> can only be enabled if the system allows it. Main issue with >> fuse-io-uring is that the mount point hangs until queues are >> initialized. If system wide io-uring is disabled queues cannot >> be initialized and the mount will hang till forcefully umounted. >> Libfuse solves that by setting up the ring before replying >> to FUSE_INIT, but we also have to consider other implementations >> and might get easily missed in development. >> >> When mount specifies user_id and group_id (e.g., via unprivileged >> fusermount with s-bit) not equal 0, the permission check must use >> the daemon's credentials, not the mount task's (root) credentials. >> Otherwise io_uring_allowed() incorrectly allows io_uring due to >> root's CAP_SYS_ADMIN capability. > > Rather than need various heuristics, it'd be a lot better if asking for > fuse-io_uring would just not "hang" at mount time and be able to recover > better? We can consider this as well. Issue is that fuse has a limit on background requests that is protected with a lock. And there is lock order to handle. Initially I didn't have this hanging mount, until I handled this background request limit in fuse-io-uring with the lock order. I.e. when one switches from /dev/fuse read/write to io-uring lock order changes. A way to avoid that issue is to split the background request limit equally between queues. Although I wouldn't like to do that before fallback to other queues is possible - which brings its own discussion points https://lore.kernel.org/r/20251003-reduced-nr-ring-queues_3-v2-0-742ff1a8fc58@ddn.com > > There are also other considerations that may mean that part of init will > fail, doesn't seem like the best idea to me to attempt to catch all of > this rather than just be able to gracefully handle errors at > initialization time. It is still doesn't seem to be right to me that fuse advertizes io-uring in FUSE_INIT to the daemon, when system wide io-uring is disabled. Thanks, Bernd