RE: [PATCH] init: Add bdev fs printk if mount_block_root failed

[Angus Chen]

> -----Original Message-----
> From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Sent: Friday, May 19, 2023 4:36 AM
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Angus Chen
> <angus.chen@xxxxxxxxxxxxxxx>
> Cc: masahiroy@xxxxxxxxxx; vbabka@xxxxxxx; peterz@xxxxxxxxxxxxx;
> paulmck@xxxxxxxxxx; rppt@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Al Viro
> <viro@xxxxxxxxxxxxxxxxxx>
> Subject: Re: [PATCH] init: Add bdev fs printk if mount_block_root failed
> 
> On Thu, May 18, 2023 at 1:02 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> wrote:
> >
> > On Thu, 18 May 2023 11:53:21 +0800 Angus Chen
> <angus.chen@xxxxxxxxxxxxxxx> wrote:
> >
> > > Attempt to printk all bdev fstype as root gives the following kernel panic:
> > >
> > > [    1.729006] VFS: Cannot open root device "vda" or
> unknown-block(253,0): error -19
> > > [    1.730603] Please append a correct "root=" boot option; here are the
> available partitions:
> > > [    1.732323] fd00          256000 vda
> > > [    1.732329]  driver: virtio_blk
> > > [    1.734194] Kernel panic - not syncing: VFS: Unable to mount root fs on
> unknown-block(253,0)
> > > [    1.734771] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.4.0-rc2+ #53
> > > [    1.735194] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
> BIOS 1.10.2-1ubuntu1 04/01/2014
> > > [    1.735772] Call Trace:
> > > [    1.735950]  <TASK>
> > > [    1.736113]  dump_stack_lvl+0x32/0x50
> > > [    1.736367]  panic+0x108/0x310
> > > [    1.736570]  mount_block_root+0x161/0x310
> > > [    1.736849]  ? rdinit_setup+0x40/0x40
> > > [    1.737088]  prepare_namespace+0x10c/0x180
> > > [    1.737393]  kernel_init_freeable+0x354/0x450
> > > [    1.737707]  ? rest_init+0xd0/0xd0
> > > [    1.737945]  kernel_init+0x16/0x130
> > > [    1.738196]  ret_from_fork+0x1f/0x30
> > >
> > > QEMU command line:
> > > "qemu-system-x86_64 -append root=/dev/vda rootfstype=ext4 ..."
> > >
> > > This error is because ext4 is not buildin and request ext4 module fail.
> 
> Cool! I'm glad this got picked up; I personally find it confusing when
> trying to start from something like an allnoconfig build then start
> enabling configs then hitting this panic; it's unclear to users that
> they are missing the config for the FS they are trying to load and on
> first glance looks like something much worse is going wrong.  In that
> sense, I view this as a win. Thanks for the patch!
Thank you, you are welcome.
> 
> Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> 
> I wish the commit message showed an example of the panic after the
> patch, to contrast the before vs. after.
en ,Did a need to send a v2 or just let it go this time?
The information is print:
[    1.570841] VFS: Cannot open root device "vda" or unknown-block(253,0): error -19
[    1.571513] Please append a correct "root=" boot option; here are the available partitions:
[    1.572056] fd00          256000 vda
[    1.572058]  driver: virtio_blk
[    1.572525] List of all bdev filesystem:
[    1.572780]  ext2
[    1.572781]
[    1.573027] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(253,0)

Or:
[    1.510467] VFS: Cannot open root device "vda" or unknown-block(253,0): error -19
[    1.512033] Please append a correct "root=" boot option; here are the available partitions:
[    1.513724] fd00          256000 vda
[    1.513731]  driver: virtio_blk
[    1.514280] Can't find any bdev filesystem!
[    1.514540] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(253,0)
> 
> > >
> > > As a hint, printk all the bdev fstype available for prompts.
> > >
> >
> > Seems reasonable.  I reworded the changelog a bit:
> >
> > : Booting with the QEMU command line:
> > : "qemu-system-x86_64 -append root=/dev/vda rootfstype=ext4 ..."
> > : will panic if ext4 is not builtin and a request to load the ext4 module
> > : fails.
> > :
> > : [    1.729006] VFS: Cannot open root device "vda" or unknown-block(253,0):
> error -19
> > : [    1.730603] Please append a correct "root=" boot option; here are the
> available partitions:
> > : [    1.732323] fd00          256000 vda
> > : [    1.732329]  driver: virtio_blk
> > : [    1.734194] Kernel panic - not syncing: VFS: Unable to mount root fs on
> unknown-block(253,0)
> > : [    1.734771] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.4.0-rc2+ #53
> > : [    1.735194] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
> BIOS 1.10.2-1ubuntu1 04/01/2014
> > : [    1.735772] Call Trace:
> > : [    1.735950]  <TASK>
> > : [    1.736113]  dump_stack_lvl+0x32/0x50
> > : [    1.736367]  panic+0x108/0x310
> > : [    1.736570]  mount_block_root+0x161/0x310
> > : [    1.736849]  ? rdinit_setup+0x40/0x40
> > : [    1.737088]  prepare_namespace+0x10c/0x180
> > : [    1.737393]  kernel_init_freeable+0x354/0x450
> > : [    1.737707]  ? rest_init+0xd0/0xd0
> > : [    1.737945]  kernel_init+0x16/0x130
> > : [    1.738196]  ret_from_fork+0x1f/0x30
> > :
> > : As a hint, print all the bdev fstypes which are available.
> >
> > > --- a/init/do_mounts.c
> > > +++ b/init/do_mounts.c
> > > @@ -427,8 +427,19 @@ void __init mount_block_root(char *name, int
> flags)
> > >               printk("VFS: Cannot open root device \"%s\" or %s:
> error %d\n",
> > >                               root_device_name, b, err);
> > >               printk("Please append a correct \"root=\" boot option;
> here are the available partitions:\n");
> > > -
> > >               printk_all_partitions();
> > > +
> > > +             if (root_fs_names)
> > > +                     num_fs = list_bdev_fs_names(fs_names,
> PAGE_SIZE);
> > > +             if (!num_fs)
> > > +                     pr_err("Can't find any bdev filesystem to be used
> for mount!\n");
> > > +             else {
> > > +                     pr_err("List of all bdev filesystem:\n");
> > > +                     for (i = 0, p = fs_names; i < num_fs; i++, p +=
> strlen(p)+1)
> > > +                             pr_err(" %s", p);
> > > +                     pr_err("\n");
> > > +             }
> > > +
> > >               panic("VFS: Unable to mount root fs on %s", b);
> > >       }
> > >       if (!(flags & SB_RDONLY)) {
> >
> > And I added a little fix.
> >
> > --- a/init/do_mounts.c~init-add-bdev-fs-printk-if-mount_block_root-failed-fix
> > +++ a/init/do_mounts.c
> > @@ -434,7 +434,7 @@ retry:
> >                 if (!num_fs)
> >                         pr_err("Can't find any bdev filesystem to be
> used for mount!\n");
> >                 else {
> > -                       pr_err("List of all bdev filesystem:\n");
> > +                       pr_err("List of all bdev filesystems:\n");
> >                         for (i = 0, p = fs_names; i < num_fs; i++, p +=
> strlen(p)+1)
> >                                 pr_err(" %s", p);
> >                         pr_err("\n");
> > _
> >
> >
> > This function now uses a jumble of printk() and pr_err().  Perhaps
> > someone will go through and rationalize all of this sometime.
> >
> 
> 
> --
> Thanks,
> ~Nick Desaulniers