Re: [RFC 0/2] ceph: add debugfs entries signifying new mount syntax support

[Jeff Layton <jlayton@redhat.com>]

On Wed, 2021-08-18 at 11:31 +0530, Venky Shankar wrote:
> [This is based on top of new mount syntax series]
> 
> Patrick proposed the idea of having debugfs entries to signify if
> kernel supports the new (v2) mount syntax. The primary use of this
> information is to catch any bugs in the new syntax implementation.
> 
> This would be done as follows::
> 
> The userspace mount helper tries to mount using the new mount syntax
> and fallsback to using old syntax if the mount using new syntax fails.
> However, a bug in the new mount syntax implementation can silently
> result in the mount helper switching to old syntax.
> 

Is this a known bug you're talking about or are you just speculating
about the potential for bugs there?

> So, the debugfs entries can be relied upon by the mount helper to
> check if the kernel supports the new mount syntax. Cases when the
> mount using the new syntax fails, but the kernel does support the
> new mount syntax, the mount helper could probably log before switching
> to the old syntax (or fail the mount altogether when run in test mode).
> 
> Debugfs entries are as follows::
> 
>     /sys/kernel/debug/ceph/
>     ....
>     ....
>     /sys/kernel/debug/ceph/dev_support
>     /sys/kernel/debug/ceph/dev_support/v2
>     ....
>     ....
> 
> Note that there is no entry signifying v1 mount syntax. That's because
> the kernel still supports mounting with old syntax and older kernels do
> not have debug entries for the same.
> 
> Venky Shankar (2):
>   ceph: add helpers to create/cleanup debugfs sub-directories under
>     "ceph" directory
>   ceph: add debugfs entries for v2 (new) mount syntax support
> 
>  fs/ceph/debugfs.c            | 28 ++++++++++++++++++++++++++++
>  fs/ceph/super.c              |  3 +++
>  fs/ceph/super.h              |  2 ++
>  include/linux/ceph/debugfs.h |  3 +++
>  net/ceph/debugfs.c           | 26 ++++++++++++++++++++++++--
>  5 files changed, 60 insertions(+), 2 deletions(-)
> 

I'm not a huge fan of this approach overall as it requires that you have
access to debugfs, and that's not guaranteed to be available everywhere.
If you want to add this for debugging purposes, that's fine, but I don't
think you want the mount helper to rely on this infrastructure.

-- 
Jeff Layton <jlayton@redhat.com>