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. --D
