Attempt to mount 9p file system as root gives the following kernel panic: 9pnet_virtio: no channels available for device root Kernel panic - not syncing: VFS: Unable to mount root "root" (9p), err=-2 CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc1+ #127 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014 Call Trace: dump_stack_lvl+0x45/0x59 panic+0x1e2/0x44b ? __warn_printk+0xf3/0xf3 ? free_unref_page+0x2d4/0x4a0 ? trace_hardirqs_on+0x32/0x120 ? free_unref_page+0x2d4/0x4a0 mount_root+0x189/0x1e0 prepare_namespace+0x136/0x165 kernel_init_freeable+0x3b8/0x3cb ? rest_init+0x2e0/0x2e0 kernel_init+0x19/0x130 ret_from_fork+0x1f/0x30 Kernel Offset: disabled ---[ end Kernel panic - not syncing: VFS: Unable to mount root "root" (9p), err=-2 ]--- QEMU command line: "qemu-system-x86_64 -append root=/dev/root rw rootfstype=9p rootflags=trans=virtio ..." This error is because root_device_name is truncated in prepare_namespace() from being "/dev/root" to be "root" prior to call to mount_nodev_root(). As a solution, don't treat errors in mount_nodev_root() as errors that require panics and allow failback to the mount flow that existed before patch citied in Fixes tag.
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| @@ -558,9 +558,6 @@ static int __init mount_nodev_root(void) · 557 unmodified lines | |||
| 558 | root_mount_data); | 558 | root_mount_data); |
| 559 | if (!err) | 559 | if (!err) |
| 560 | break; | 560 | break; |
| 561 | if (err != -EACCES && err != -EINVAL) | ||
| 562 | panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n", | ||
| 563 | root_device_name, fstype, err); | ||
| 564 | } | 561 | } |
| 565 | 562 | ||
| 566 | free_page((unsigned long)fs_names); | 563 | free_page((unsigned long)fs_names); |