Hi,

On 26/02/2020 09:11, Miklos Szeredi wrote:

On Tue, Feb 25, 2020 at 4:29 PM James Bottomley
[off-list ref] wrote:

quoted

The other thing a file descriptor does that sysfs doesn't is that it
solves the information leak: if I'm in a mount namespace that has no
access to certain mounts, I can't fspick them and thus I can't see the
information.  By default, with sysfs I can.

That's true, but procfs/sysfs has to deal with various namespacing
issues anyway.  If this is just about hiding a number of entries, then
I don't think that's going to be a big deal.

The syscall API is efficient: single syscall per query instead of
several, no parsing necessary.

However, it is difficult to extend, because the ABI must be updated,
possibly libc and util-linux also, so that scripts can also consume
the new parameter.  With the sysfs approach only the kernel needs to
be updated, and possibly only the filesystem code, not even the VFS.

So I think the question comes down to:  do we need a highly efficient
way to query the superblock parameters all at once, or not?

Thanks,
Miklos

That is Ian's use case for autofs I think, and it will also be what is 
needed at start up of most applications using the fs notifications, as 
well as at resync time if there has been an overrun leading to lost fs 
notification messages. We do need a solution that can scale to large 
numbers of mounts efficiently. Being able to extend it is also an 
important consideration too, so hopefully David has a solution to that,

Steve.