Re: [RFC] Another take at restarting FUSE servers

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

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.

All of this of course relies on fuse2fs maintaining as little volatile
state of its own as possible.  I think that means disabling the block
cache in the unix io manager, and if we ever implemented delalloc then
either we'd have to save the reservations somewhere or I guess you could
immediately syncfs the whole filesystem to try to push all the dirty
data to disk before we start allowing new free space allocations for new
changes.

--D

>      	     	      		     	     - Ted
>