Re: [PATCH 34/38] vfs: syscall: Add fsinfo() to query filesystem information [ver #10]

David Howells
2018-07-31T12:39:21Z

Al Viro <viro@ZenIV.linux.org.uk> wrote:

> Umm... What's so special about cell/volume/domain/realm?

Nothing particularly. But they're something various network filesystems might
find useful. cell for AFS, domain for CIFS, realm for things that use
kerberos.

volume_id/uuid/name would be usable by ext4 too, for example.

> And what do we do when a random filesystem gets added - should its
> parameters go into catch-all pile (attr_parameter),

FSINFO_ATTR_PARAMETER is a way to enumerate the configuration parameters
passed to mount, as an alternative to parsing /proc/mounts. So, for example,
afs has:

enum afs_param {
Opt_autocell,
Opt_dyn,
Opt_source,
nr__afs_params
};

static const struct fs_parameter_spec afs_param_specs[nr__afs_params] = {
[Opt_autocell] = { fs_param_takes_no_value },
[Opt_dyn] = { fs_param_takes_no_value },
[Opt_source] = { fs_param_is_string },
};

static const struct constant_table afs_param_keys[] = {
{ "autocell", Opt_autocell },
{ "dyn", Opt_dyn },
{ "source", Opt_source },
};

My thought is that calling fsinfo(..., "/some/afs/file", &params, ...) with:

struct fsinfo_params params = {
.request = FSINFO_ATTR_PARAMETER,
.Nth = <parameter-number>,
};

would get you back, for example:

Nth Result
======= ==========================================
0 "autocell" (or "" if not set)
1 "dyn" (or "" if not set)
2 "source=%#grand.central.org:root.cell."
3+ -ENODATA (ie. there are no more)

where Nth corresponds to the parameter specified by
FSINFO_ATTR_PARAM_DESCRIPTION and Nth.

Now for some filesystems, cgroups-v1 for example, there are parameters beyond
the list (the subsystem name) and these can be listed after the predefined
parameters, eg.:

Nth Result
======= ==========================================
0 "all" or ""
1 "clone_children" or ""
2 "cpuset_v2_mode" or ""
3 "name" or ""
4 "none" or ""
5 "noprefix" or ""
6 "release_agent" or ""
7 "xattr" or ""
8 "<subsys0>" or ""
9 "<subsys1>" or ""
10 "<subsys2>" or ""
... -ENODATA

> or should they get classes of their own?

Yes.

> For Cthulhu sake, who's going to maintain that enum in face of
> random out-of-tree filesystems, each wanting a class or two its own?

They don't get their own numbers unless they're in-tree. Full stop. We have
the same issue with system calls and not-yet-upstream new syscalls.

Note that, as I have the code now, the "type" of return value for each
attribute must also be declared to the fsinfo() core, and the fsinfo core does
the copy to/from userspace.

> We'd tried that with device numbers; ask hpa how well has that
> worked and how much did he love the whole experience...

What would you do instead? I would prefer to avoid using text strings as keys
because then I need a big lookup table, and possibly this gets devolved to
each filesystem to handle - which ends up even more of a mess because then
there's nothing to hold consistency.

David