On 13/01/2021 19:00, David Sterba wrote:

so what changed that it's fine to use device->fs_info now while it was
not before?

The time when we call btrfs_get_dev_zone_info() in "btrfs: defer loading
zone info after opening trees". 

From the commit message:

"The current call site of btrfs_get_dev_zone_info() during the mount process
is earlier than reading the trees, so we can't slice a regular device to
conventional zones. This patch defers the loading of zone info to
open_ctree() to load the emulated zone size from a device extent."
    
Hope this clarifies your concerns.