Device for root volume appears as '/dev/root', 7.3-rc1 regression

[Hanabishi <i.r.e.c.c.a.k.u.n+kernel.org@gmail.com>]

Hello.

I initially reported this in https://lore.kernel.org/linux-btrfs/018a9738-1d4a-43a0-9352-a56d1e541364@gmail.com/, but haven't got any ack, so assume my message was overlooked.
That patch introduced a regression for me in 7.3-rc1. I'll duplicate full report here.

Device for root volume appears as '/dev/root' now instead of the actual '/dev/sdX' device, but the '/dev/root' path itself doesn't exist.
That causes confusion for userspace applications and leads to variety of problems, like

udisksd[1728]: Error statting /dev/root: No such file or directory

Interesting that the 'mount' command doesn't reflect that, but you can see it in other ways.

$ df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/root      243931136 3044560 240085792   2% /

$ cat /proc/self/mountinfo
21 1 0:19 /@arch / rw,noatime shared:1 - btrfs /dev/root rw,compress-force=zstd:3,ssd,flushoncommit,discard=async,space_cache=v2,subvolid=256,subvol=/@arch

Reverting this change (mainline commit 108cc873398932af589c295f78c348513b8d70d9) fixes the issue.

$ df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/sda2      243931136 3044336 240085984   2% /

$ cat /proc/self/mountinfo
21 1 0:19 /@arch / rw,noatime shared:1 - btrfs /dev/sda2 rw,compress-force=zstd:3,ssd,flushoncommit,discard=async,space_cache=v2,subvolid=256,subvol=/@arch