Edit: I have tried mounting the sd-card with an empty ext4-partition, and it worked just fine, but after I ran
debootstrap on the sd-card, the problem reappeared with identical symptoms.
I want to mount an ext4-formatted sd-card, but
# mount -t ext4 /dev/block/mmcblk1p1 mountpoint
mount: mounting /dev/block/mmcblk1p1 on mountpoint/ failed: Invalid argument
So far I have:
made sure the sd-card itself is OK (mounts fine on my debian laptop)
made sure the block device is the right one
busybox mount, which is a different binary, but gives the same error
The mountpoint exists, and the kernel supports ext4. My device is a rooted Samsung Galaxy Tab S2 running a Lineage OS 14.1 ROM (Android 7.1), SELinux is set to permissive.
Any pointers are appreciated!