From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 377D22C9A; Thu, 31 Jul 2025 11:33:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753961594; cv=none; b=iKIZaSICEhDCkFq/TKCFbsCFice1Pe+TISfBxivRkc5wpR0Z7kjCXg2l8yus1jzQRecTc3/c1y8so+qhCBfV0rjiHdoz6kbTgNYJMexahm0uB5Cu6ohEia9NpvzpFQaWUwKQVsV2lhByyao3QjTu6VWM+vjRCVfQUQgkqYIZIJo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753961594; c=relaxed/simple; bh=sTbx7McNSccTMzi7rESQoqeGOeBghvgV4NqCILAaT3M=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=krV+Sbyqe//1+E7z1LV91p6CGlKRR/RdLdFX2BnPcGRWg6wJ6DdvImU22m8cPYE0085w5vYivzgW/6QAh2G6dYa9e3Vkm8W6q2tTR/4f7ueLTnHR8hU+UmK6qGI57JZp8YA/rHejROS0Q/ONTgp7UlzJoV7C8hecx7I9e58vBYA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=e/qe3p+e; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="e/qe3p+e" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 25382C4CEEF; Thu, 31 Jul 2025 11:33:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1753961593; bh=sTbx7McNSccTMzi7rESQoqeGOeBghvgV4NqCILAaT3M=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=e/qe3p+eRLamN7D5azYA3q3rxP2X2iSZAfM7QwsPKP6E8FP9Yg2Nv8oesgdd0gTUT S1ZgL+2m0/CRdjMDGQJl4qYVlbJ7Ms6xuSLFxMczXq/egP8WqscurAeOHlMs0m5amy MEazmBmhAIK9DVgJSBi4JkJEpIZ/ozykpJAQJixkeKNijlMmK5NFEdPrMnkYbZivJf wUnO3nm65BPSCDjrmPt95WU80G7p6nPChf97RLIepkPaWqB8mjx9d4GXOANIkpPaeU KTwQjdavFgRJV3sBYBWcIvVdUwfrutgOTc8bFqi0wCnfQfiZOK4EfIDCTaLg+WO2C+ BZsgjIxE10qZg== Date: Thu, 31 Jul 2025 13:33:09 +0200 From: Christian Brauner To: Luis Henriques Cc: "Darrick J. Wong" , Miklos Szeredi , Bernd Schubert , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC] Another take at restarting FUSE servers Message-ID: <20250731-diamant-kringeln-7f16e5e96173@brauner> References: <8734afp0ct.fsf@igalia.com> <20250729233854.GV2672029@frogsfrogsfrogs> <87freddbcf.fsf@igalia.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87freddbcf.fsf@igalia.com> On Wed, Jul 30, 2025 at 03:04:00PM +0100, Luis Henriques wrote: > Hi Darrick, > > On Tue, Jul 29 2025, Darrick J. Wong wrote: > > > On Tue, Jul 29, 2025 at 02:56:02PM +0100, Luis Henriques wrote: > >> Hi! > >> > >> I know this has been discussed several times in several places, and the > >> recent(ish) addition of NOTIFY_RESEND is an important step towards being > >> able to restart a user-space FUSE server. > >> > >> While looking at how to restart a server that uses the libfuse lowlevel > >> API, I've created an RFC pull request [1] to understand whether adding > >> support for this operation would be something acceptable in the project. > > > > Just speaking for fuse2fs here -- that would be kinda nifty if libfuse > > could restart itself. It's unclear if doing so will actually enable us > > to clear the condition that caused the failure in the first place, but I > > suppose fuse2fs /does/ have e2fsck -fy at hand. So maybe restarts > > aren't totally crazy. > > Maybe my PR lacks a bit of ambition -- it's goal wasn't to have libfuse do > the restart itself. Instead, it simply adds some visibility into the > opaque data structures so that a FUSE server could re-initialise a session > without having to go through a full remount. > > But sure, there are other things that could be added to the library as > well. For example, in my current experiments, the FUSE server needs start > some sort of "file descriptor server" to keep the fd alive for the > restart. This daemon could be optionally provided in libfuse itself, > which could also be used to store all sorts of blobs needed by the file > system after recovery is done. Fwiw, for most use-cases you really just want to use systemd's file descriptor store to persist the /dev/fuse connection: https://systemd.io/FILE_DESCRIPTOR_STORE/ > > >> The PR doesn't do anything sophisticated, it simply hacks into the opaque > >> libfuse data structures so that a server could set some of the sessions' > >> fields. > >> > >> So, a FUSE server simply has to save the /dev/fuse file descriptor and > >> pass it to libfuse while recovering, after a restart or a crash. The > >> mentioned NOTIFY_RESEND should be used so that no requests are lost, of > >> course. And there are probably other data structures that user-space file > >> systems will have to keep track as well, so that everything can be > >> restored. (The parameters set in the INIT phase, for example.) > > > > Yeah, I don't know how that would work in practice. Would the kernel > > send back the old connection flags and whatnot via some sort of > > FUSE_REINIT request, and the fuse server can either decide that it will > > try to recover, or just bail out? > > That would be an option. But my current idea would be that the server > would need to store those somewhere and simply assume they are still OK The fdstore currently allows to associate a name with a file descriptor in the fdstore. That name would allow you to associate the options with the fuse connection. However, I would not rule it out that additional metadata could be attached to file descriptors in the fdstore if that's something that's needed.