Commit 6f861765464f ("fs: Block writes to mounted block devices") in the 6.8 upstream kernel prevents writing to devices that are mounted as filesystems.

It seems that gfs2_grow does exactly that for writing the additional resource group metadata onto disk. We need to make sure that the kernel either allows those writes to succeed, or else implement writing the additional resource group metadata in the kernel.

We should probably ask Jan Kara (the author of commit 6f861765464f) about his thoughts on dealing with these kinds of problems.

tunegfs2 will probably also not work on mounted devices anymore, but using it that way is discouraged and unnecessary, anyway.

gfs2_jadd probably isn't affected because it creates normal files and moves them into place in the "gfs2meta" filesystem.