Hi Alekséi,

On 2025/12/26 20:17, Alekséi Naidénov wrote:

Hello,

I am reporting a regression in the 6.12 stable series related to EROFS file-backed mounts.

After updating from Linux 6.12.62 to 6.12.63, a previously working setup using OSTree-backed composefs mounts as Podman rootfs no longer works.

The regression appears to be caused by the following commit:

34447aeedbaea8f9aad3da5b07030a1c0e124639 ("erofs: limit the level of fs stacking for file-backed mounts") (backport of upstream commit d53cd891f0e4311889349fff3a784dc552f814b9)

## Setup description

We use OSTree to materialize filesystem trees, which are mounted via composefs (EROFS + overlayfs) as a read-only filesystem. This mounted composefs tree is then used as a Podman rootfs, with Podman mounting a writable overlayfs on top for each container.

This setup worked correctly on Linux 6.12.62 and earlier.

The following issue just tracks this: https://github.com/coreos/fedora-coreos-tracker/issues/2087

I don't think more information is needed, but I really think the EROFS commit is needed to avoid kernel stack overflow due to nested fses.

In short, the stacking looks like:

EROFS (file-backed) -> composefs (EROFS + overlayfs with ostree repo as datadir, read-only) -> Podman rootfs overlays (RW upperdir)

There is no recursive or self-stacking of EROFS.

Yes, but there are two overlayfs + one file-backed EROFS already, and it exceeds FILESYSTEM_MAX_STACK_DEPTH.

That is overlayfs refuses to mount the nested fses.

Thanks, Gao Xiang