Re: [PATCH 2/7] fuse: flush pending fuse events before aborting the connection

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jul 21, 2025 at 01:05:02PM -0700, Joanne Koong wrote:
> On Sat, Jul 19, 2025 at 12:18 AM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Sat, Jul 19, 2025 at 12:23 AM Joanne Koong <joannelkoong@gmail.com> wrote:
> > >
> > > On Thu, Jul 17, 2025 at 4:26 PM Darrick J. Wong <djwong@kernel.org> wrote:
> > > >
> > > > From: Darrick J. Wong <djwong@kernel.org>
> > > >
> > > > generic/488 fails with fuse2fs in the following fashion:
> > > >
> > > > Unfortunately, the 488.full file shows that there are a lot of hidden
> > > > files left over in the filesystem, with incorrect link counts.  Tracing
> > > > fuse_request_* shows that there are a large number of FUSE_RELEASE
> > > > commands that are queued up on behalf of the unlinked files at the time
> > > > that fuse_conn_destroy calls fuse_abort_conn.  Had the connection not
> > > > aborted, the fuse server would have responded to the RELEASE commands by
> > > > removing the hidden files; instead they stick around.
> > >
> > > Tbh it's still weird to me that FUSE_RELEASE is asynchronous instead
> > > of synchronous. For example for fuse servers that cache their data and
> > > only write the buffer out to some remote filesystem when the file gets
> > > closed, it seems useful for them to (like nfs) be able to return an
> > > error to the client for close() if there's a failure committing that
> > > data; that also has clearer API semantics imo, eg users are guaranteed
> > > that when close() returns, all the processing/cleanup for that file
> > > has been completed.  Async FUSE_RELEASE also seems kind of racy, eg if
> > > the server holds local locks that get released in FUSE_RELEASE, if a
> > > subsequent FUSE_OPEN happens before FUSE_RELEASE then depends on
> > > grabbing that lock, then we end up deadlocked if the server is
> > > single-threaded.
> > >
> >
> > There is a very good reason for keeping FUSE_FLUSH and FUSE_RELEASE
> > (as well as those vfs ops) separate.
> 
> Oh interesting, I didn't realize FUSE_FLUSH gets also sent on the
> release path. I had assumed FUSE_FLUSH was for the sync()/fsync()

(That's FUSE_FSYNC)

> case. But I see now that you're right, close() makes a call to
> filp_flush() in the vfs layer. (and I now see there's FUSE_FSYNC for
> the fsync() case)

Yeah, flush-on-close (FUSE_FLUSH) is generally a good idea for
"unreliable" filesystems -- either because they're remote, or because
the local storage they're on could get yanked at any time.  It's slow,
but it papers over a lot of bugs and "bad" usage.

> > A filesystem can decide if it needs synchronous close() (not release).
> > And with FOPEN_NOFLUSH, the filesystem can decide that per open file,
> > (unless it conflicts with a config like writeback cache).
> >
> > I have a filesystem which can do very slow io and some clients
> > can get stuck doing open;fstat;close if close is always synchronous.
> > I actually found the libfuse feature of async flush (FUSE_RELEASE_FLUSH)
> > quite useful for my filesystem, so I carry a kernel patch to support it.
> >
> > The issue of racing that you mentioned sounds odd.
> > First of all, who runs a single threaded fuse server?
> > Second, what does it matter if release is sync or async,
> > FUSE_RELEASE will not be triggered by the same
> > task calling FUSE_OPEN, so if there is a deadlock, it will happen
> > with sync release as well.
> 
> If the server is single-threaded, I think the FUSE_RELEASE would have
> to happen on the same task as FUSE_OPEN, so if the release is
> synchronous, this would avoid the deadlock because that guarantees the
> FUSE_RELEASE happens before the next FUSE_OPEN.

On a single-threaded server(!) I would hope that the release would be
issued to the fuse server before the open.  (I'm not sure I understand
where this part of the thread went, because why would that happen?  And
why would the fuse server hold a lock across requests?)

> However now that you pointed out FUSE_FLUSH gets sent on the release
> path, that addresses my worry about async FUSE_RELEASE returning
> before the server has gotten a chance to write out their local buffer
> cache.

<nod>

--D

> Thanks,
> Joanne
> >
> > Thanks,
> > Amir.
>