Solving the "Could not unmount revokefs-fuse" Flatpak Error on Ubuntu after 25.10 Upgrade

If you're an Ubuntu user running Flatpak applications with an NVIDIA graphics card, you may have encountered a frustrating and persistent error message during updates:

Could not unmount revokefs-fuse filesystem at /var/tmp/flatpak-cache-[...]: Child process exited with code 1

This error can stop your updates and installations in their tracks. Even more frustrating, common solutions like rebooting your system, running flatpak repair, or trying to force unmount the directory often don't work.

Here’s a step-by-step breakdown of how we diagnosed and solved this stubborn problem.

The Problem

The error appeared when trying to install two application updates through the Discover software center. Every attempt to update would fail with the revokefs-fuse error message.

Initial Troubleshooting Steps (That Didn't Work)

Standard troubleshooting methods were the first to be tried, but none resolved the issue:

• Rebooting: The error persisted across multiple reboots.
• Force Unmount (umount): The system reported that the target mount point didn't exist, indicating the problem was happening fresh with each attempt, not due to a leftover process.
• Flatpak Maintenance: Commands like flatpak repair and flatpak uninstall --unused completed successfully but did not fix the underlying error.

The Investigation: A Common Suspect Ruled Out

Typically, this error points to a mismatch between the NVIDIA driver installed on the host system and the specific NVIDIA driver runtime Flatpak is trying to use.

The first diagnostic step was to check the system's driver version with the nvidia-smi command. The output showed:

Driver Version: 580.82.07

This matched the version Flatpak was failing on (nvidia-580-82-07) perfectly. This crucial piece of information ruled out the version mismatch theory and suggested a different root cause: the Flatpak driver runtime itself was likely corrupted.

The Solution: A Forceful Reinstallation

Since flatpak repair didn't fix the corruption, a manual, forceful reinstallation of the specific runtime was the next logical step.

Step 1: Manually Uninstall the Problematic Runtime The exact runtime mentioned in the error message was targeted for uninstallation. This is the key step to force Flatpak to fetch a fresh copy.

flatpak uninstall org.freedesktop.Platform.GL.nvidia-580-82-07

Step 2: Clear the Temporary Cache To ensure no corrupted temporary files were left behind, the Flatpak cache directory was manually cleared.

sudo rm -rf /var/tmp/flatpak-cache-*

Step 3: Trigger the Reinstallation By simply running the original update again (or flatpak update in the terminal), Flatpak detected that a required dependency—the NVIDIA driver—was now missing. It prompted to install the driver, and this time, it downloaded a fresh, uncorrupted copy from the repository.

The Result

After the new driver runtime was successfully installed, the revokefs-fuse error vanished.

Interestingly, the two application updates that had been stuck in Discover also disappeared. This confirmed that they were dependent on the NVIDIA runtime, and their updates were blocked by the corrupted driver. Once the core dependency was fixed, the application updates were able to complete successfully in the background.

TL;DR

If you're stuck on the Could not unmount revokefs-fuse error and have confirmed your NVIDIA driver versions match, the Flatpak runtime itself is likely corrupted. The solution is to force a reinstallation:

1. Use flatpak uninstall to remove the specific org.freedesktop.Platform.GL.nvidia-* package that's causing the error.
2. Clear the temporary cache with sudo rm -rf /var/tmp/flatpak-cache-*.
3. Run flatpak update to trigger a fresh download and installation of the driver runtime.

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