Message-ID: <CAOQ4uxhLM11Zq9P=E1VyN7puvBs80v0HrPU6HqY0LLM6HVc_ZQ@mail.gmail.com>
Date: Tue, 16 Sep 2025 09:59:36 +0200
From: Amir Goldstein <amir73il@...il.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Bernd Schubert <bernd@...ernd.com>, Luis Henriques <luis@...lia.com>, "Theodore Ts'o" <tytso@....edu>, 
	Miklos Szeredi <miklos@...redi.hu>, Bernd Schubert <bschubert@....com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Kevin Chen <kchen@....com>
Subject: Re: [RFC] Another take at restarting FUSE servers

On Tue, Sep 16, 2025 at 4:53 AM Darrick J. Wong <djwong@...nel.org> wrote:
>
> On Mon, Sep 15, 2025 at 10:41:31AM +0200, Amir Goldstein wrote:
> > On Mon, Sep 15, 2025 at 10:27 AM Bernd Schubert <bernd@...ernd.com> wrote:
> > >
> > >
> > >
> > > On 9/15/25 09:07, Amir Goldstein wrote:
> > > > On Fri, Sep 12, 2025 at 4:58 PM Darrick J. Wong <djwong@...nel.org> wrote:
> > > >>
> > > >> On Fri, Sep 12, 2025 at 02:29:03PM +0200, Bernd Schubert wrote:
> > > >>>
> > > >>>
> > > >>> On 9/12/25 13:41, Amir Goldstein wrote:
> > > >>>> On Fri, Sep 12, 2025 at 12:31 PM Bernd Schubert <bernd@...ernd.com> wrote:
> > > >>>>>
> > > >>>>>
> > > >>>>>
> > > >>>>> On 8/1/25 12:15, Luis Henriques wrote:
> > > >>>>>> On Thu, Jul 31 2025, Darrick J. Wong wrote:
> > > >>>>>>
> > > >>>>>>> On Thu, Jul 31, 2025 at 09:04:58AM -0400, Theodore Ts'o wrote:
> > > >>>>>>>> On Tue, Jul 29, 2025 at 04:38:54PM -0700, Darrick J. Wong wrote:
> > > >>>>>>>>>
> > > >>>>>>>>> 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.
> > > >>>>>>>>
> > > >>>>>>>> I'm trying to understand what the failure scenario is here.  Is this
> > > >>>>>>>> if the userspace fuse server (i.e., fuse2fs) has crashed?  If so, what
> > > >>>>>>>> is supposed to happen with respect to open files, metadata and data
> > > >>>>>>>> modifications which were in transit, etc.?  Sure, fuse2fs could run
> > > >>>>>>>> e2fsck -fy, but if there are dirty inode on the system, that's going
> > > >>>>>>>> potentally to be out of sync, right?
> > > >>>>>>>>
> > > >>>>>>>> What are the recovery semantics that we hope to be able to provide?
> > > >>>>>>>
> > > >>>>>>> <echoing what we said on the ext4 call this morning>
> > > >>>>>>>
> > > >>>>>>> With iomap, most of the dirty state is in the kernel, so I think the new
> > > >>>>>>> fuse2fs instance would poke the kernel with FUSE_NOTIFY_RESTARTED, which
> > > >>>>>>> would initiate GETATTR requests on all the cached inodes to validate
> > > >>>>>>> that they still exist; and then resend all the unacknowledged requests
> > > >>>>>>> that were pending at the time.  It might be the case that you have to
> > > >>>>>>> that in the reverse order; I only know enough about the design of fuse
> > > >>>>>>> to suspect that to be true.
> > > >>>>>>>
> > > >>>>>>> Anyhow once those are complete, I think we can resume operations with
> > > >>>>>>> the surviving inodes.  The ones that fail the GETATTR revalidation are
> > > >>>>>>> fuse_make_bad'd, which effectively revokes them.
> > > >>>>>>
> > > >>>>>> Ah! Interesting, I have been playing a bit with sending LOOKUP requests,
> > > >>>>>> but probably GETATTR is a better option.
> > > >>>>>>
> > > >>>>>> So, are you currently working on any of this?  Are you implementing this
> > > >>>>>> new NOTIFY_RESTARTED request?  I guess it's time for me to have a closer
> > > >>>>>> look at fuse2fs too.
> > > >>>>>
> > > >>>>> Sorry for joining the discussion late, I was totally occupied, day and
> > > >>>>> night. Added Kevin to CC, who is going to work on recovery on our
> > > >>>>> DDN side.
> > > >>>>>
> > > >>>>> Issue with GETATTR and LOOKUP is that they need a path, but on fuse
> > > >>>>> server restart we want kernel to recover inodes and their lookup count.
> > > >>>>> Now inode recovery might be hard, because we currently only have a
> > > >>>>> 64-bit node-id - which is used my most fuse application as memory
> > > >>>>> pointer.
> > > >>>>>
> > > >>>>> As Luis wrote, my issue with FUSE_NOTIFY_RESEND is that it just re-sends
> > > >>>>> outstanding requests. And that ends up in most cases in sending requests
> > > >>>>> with invalid node-IDs, that are casted and might provoke random memory
> > > >>>>> access on restart. Kind of the same issue why fuse nfs export or
> > > >>>>> open_by_handle_at doesn't work well right now.
> > > >>>>>
> > > >>>>> So IMHO, what we really want is something like FUSE_LOOKUP_FH, which
> > > >>>>> would not return a 64-bit node ID, but a max 128 byte file handle.
> > > >>>>> And then FUSE_REVALIDATE_FH on server restart.
> > > >>>>> The file handles could be stored into the fuse inode and also used for
> > > >>>>> NFS export.
> > > >>>>>
> > > >>>>> I *think* Amir had a similar idea, but I don't find the link quickly.
> > > >>>>> Adding Amir to CC.
> > > >>>>
> > > >>>> Or maybe it was Miklos' idea. Hard to keep track of this rolling thread:
> > > >>>> https://lore.kernel.org/linux-fsdevel/CAJfpegvNZ6Z7uhuTdQ6quBaTOYNkAP8W_4yUY4L2JRAEKxEwOQ@mail.gmail.com/
> > > >>>
> > > >>> Thanks for the reference Amir! I even had been in that thread.
> > > >>>
> > > >>>>
> > > >>>>>
> > > >>>>> Our short term plan is to add something like FUSE_NOTIFY_RESTART, which
> > > >>>>> will iterate over all superblock inodes and mark them with fuse_make_bad.
> > > >>>>> Any objections against that?
> > > >>
> > > >> What if you actually /can/ reuse a nodeid after a restart?  Consider
> > > >> fuse4fs, where the nodeid is the on-disk inode number.  After a restart,
> > > >> you can reconnect the fuse_inode to the ondisk inode, assuming recovery
> > > >> didn't delete it, obviously.
> > > >
> > > > FUSE_LOOKUP_HANDLE is a contract.
> > > > If fuse4fs can reuse nodeid after restart then by all means, it should sign
> > > > this contract, otherwise there is no way for client to know that the
> > > > nodeids are persistent.
> > > > If fuse4fs_handle := nodeid, that will make implementing the lookup_handle()
> > > > API trivial.
> > > >
> > > >>
> > > >> I suppose you could just ask for refreshed stat information and either
> > > >> the server gives it to you and the fuse_inode lives; or the server
> > > >> returns ENOENT and then we mark it bad.  But I'd have to see code
> > > >> patches to form a real opinion.
> > > >>
> > > >
> > > > You could make fuse4fs_handle := <nodeid:fuse_instance_id>
> > > > where fuse_instance_id can be its start time or random number.
> > > > for auto invalidate, or maybe the fuse_instance_id should be
> > > > a native part of FUSE protocol so that client knows to only invalidate
> > > > attr cache in case of fuse_instance_id change?
> > > >
> > > > In any case, instead of a storm of revalidate messages after
> > > > server restart, do it lazily on demand.
> > >
> > > For a network file system, probably. For fuse4fs or other block
> > > based file systems, not sure. Darrick has the example of fsck.
> > > Let's assume fuse4fs runs with attribute and dentry timeouts > 0,
> > > fuse-server gets restarted, fsck'ed and some files get removed.
> > > Now reading these inodes would still work - wouldn't it
> > > be better to invalidate the cache before going into operation
> > > again?
> >
> > Forgive me, I was making a wrong assumption that fuse4fs
> > was using ext4 filehandle as nodeid, but of course it does not.
>
> Well now that you mention it, there /is/ a risk of shenanigans like
> that.  Consider:
>
> 1) fuse4fs mount an ext4 filesystem
> 2) crash the fuse4fs server
> <fuse4fs server restart stalls...>
> 3) e2fsck -fy /dev/XXX deletes inode 17
> 4) someone else mounts the fs, makes some changes that result in 17
>    being reallocated, user says "OOOOOPS", unmounts it
> 5) fuse4fs server finally restarts, and reconnects to the kernel
>
> Hey, inode 17 is now a different file!!
>
> So maybe the nodeid has to be an actual file handle.  Oh wait, no,
> everything's (potentially) fine because fuse4fs supplied i_generation to
> the kernel, and fuse_stale_inode will mark it bad if that happens.
>
> Hm ok then, at least there's a way out. :)
>

Right.

> > The reason I made this wrong assumption is because fuse4fs *can*
> > already use ext4 (64bit) file handle as nodeid, with existing FUSE protocol
> > which is what my fuse passthough library [1] does.
> >
> > My claim was that although fuse4fs could support safe restart, which
> > cannot read from recycled inode number with current FUSE protocol,
> > doing so with FUSE_HANDLE protocol would express a commitment
>
> Pardon my naïvete, but what is FUSE_HANDLE?
>
> $ git grep -w FUSE_HANDLE fs
> $

Sorry, braino. I meant LOOKUP_HANDLE (or FUSE_LOOKUP_HANDLE):
https://lore.kernel.org/linux-fsdevel/CAJfpegvNZ6Z7uhuTdQ6quBaTOYNkAP8W_4yUY4L2JRAEKxEwOQ@mail.gmail.com/

Which means to communicate a variable sized "nodeid"
which can also be declared as an object id that survives server restart.

Basically, the reason that I brought up LOOKUP_HANDLE is to
properly support NFS export of fuse filesystems.

My incentive was to support a proper fuse server restart/remount/re-export
with the same fsid in /etc/exports, but this gives us a better starting point
for fuse server restart/re-connect.

Thanks,
Amir.

Powered by blists - more mailing lists