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 005842F30; Mon, 4 Aug 2025 08:45:48 +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=1754297149; cv=none; b=aDIwjPXBqTDLznv5IOhGE76TRZEqTRDahHormxMhPdhwd2r0JCnGql3bgDdEsPn23xLMyBeLNen8YmVwO2fCNcak02JyjwyhCdeIFmAp0abEXRT9DhslyNUdlr9dtR+8++rfaYDg4bF/KODWJEECMRmlDJtlKQ4hQvEAtUpfmlY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754297149; c=relaxed/simple; bh=uuDHiYwQIffCG6IrU7+9PPyyyGEjkOwbsx1qo2fJD2I=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=BpSqfzdTfAKZOpiWY50uzlY0QvuvJQRJSWWmURYGMTYwmZH/dUAiykKjTPx+PjVy7yJx2CknDCO2R11BQ/BlYMjruCPJnxCHGftNQmND5upM2+/RuojpCTzhMbhX/aedcd2ypKa9DTCl5eRfEJmZPjWTuagpf4BApd3yPrSmK3E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MygY6za/; 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="MygY6za/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 29278C4CEE7; Mon, 4 Aug 2025 08:45:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1754297148; bh=uuDHiYwQIffCG6IrU7+9PPyyyGEjkOwbsx1qo2fJD2I=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=MygY6za/7JQ8298Ydmi7env+67ti+AqAjBZTE1/buPxLIMF9zeHwqrdNLHfMaIJ6G PdofNwBgkHccLko+bFZh39rrqDof/vNLRRAn8XhNhV8K9g/5eIqgtXGw9h3ECLxtrQ OT/HWCPs1VpNCh6Dg/7/gbOurMduy0sAUP4cqdt9UoDiHDekMAQC8CNGlgcPjsrQFZ ZZQffZXSk785CvD2FFZPgGqTG8fDBHiwCh3A3JUW7IQnI9dF5DhpcsWVyk/2G8q7me 7AEpRDoFDtfYetU3JaOXZ94Io42dvw2i+qLxEFQy9hgNRFs2ykQpUS7W7Cg/spaNCE +ZdBrDh5yrwVA== Date: Mon, 4 Aug 2025 10:45:44 +0200 From: Christian Brauner To: "Darrick J. Wong" Cc: Luis Henriques , 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: <20250804-lesezeichen-kugel-7a8b7053d236@brauner> References: <8734afp0ct.fsf@igalia.com> <20250729233854.GV2672029@frogsfrogsfrogs> <87freddbcf.fsf@igalia.com> <20250731-diamant-kringeln-7f16e5e96173@brauner> <20250731172946.GK2672070@frogsfrogsfrogs> 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: <20250731172946.GK2672070@frogsfrogsfrogs> On Thu, Jul 31, 2025 at 10:29:46AM -0700, Darrick J. Wong wrote: > On Thu, Jul 31, 2025 at 01:33:09PM +0200, Christian Brauner wrote: > > 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/ > > Very nice! This is exactly what I was looking for to handle the initial > setup, so I'm glad I don't have to go design a protocol around that. > > > > > > > >> 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. > > Names are useful, I'd at least want "fusedev", "fsopen", and "device". > > If someone passed "journal_dev=/dev/sdaX" to fuse2fs then I'd want it to > be able to tell mountfsd "Hey, can you also open /dev/sdaX and put it in > the store as 'journal_dev'?" Then it just has to wait until the fd shows > up, and it can continue with the mount process. > > Though the "device" argument needn't be a path, so to be fully general > mountfsd and the fuse server would have to handshake that as well. Fwiw, to attach arbitrary metadata to a file descriptor the easiest thing to do would be to stash both a (fuse server) file descriptor and then also a memfd via memfd_create() that e.g., can contain all the server options that you want to store.