file metadata via fs API

[Miklos Szeredi]

On Wed, Aug 12, 2020 at 1:28 PM Karel Zak <kzak at redhat.com> wrote:

> The proposal is based on paths and open(), how do you plan to deal
> with mount IDs? David's fsinfo() allows to ask for mount info by mount
> ID and it works well with mount notification where you get the ID. The
> collaboration with notification interface is critical for our use-cases.

One would use the notification to keep an up to date set of attributes
for each watched mount, right?

That presumably means the mount ID <-> mount path mapping already
exists, which means it's just possible to use the open(mount_path,
O_PATH) to obtain the base fd.

If that assumption is not true, we could add a new interface for
opening the root of the mount by ID.  Fsinfo uses the dfd as a root
for checking connectivity and the filename as the mount ID + a special
flag indicating that it's not "dfd + path" we are dealing with but
"rootfd + mntid".  That sort of semantic multiplexing is quite ugly
and I wouldn't suggest doing that with openat(2).

A new syscall that returns an fd pointing to the root of the mount
might be the best solution:

   int open_mount(int root_fd, u64 mntid, int flags);

Yeah, yeah this is adding just another syscall interface, but notice how:

 a) it does one simple thing, no multiplexing at all

 b) is general purpose, and could be used for example in conjunction
with open_by_handle_at(2), that also requires an fd pointing to a
mount.

Thanks,
Miklos