Re: file metadata via fs API

On Wed, Aug 12, 2020 at 02:43:32PM +0200, Miklos Szeredi wrote:

On Wed, Aug 12, 2020 at 1:28 PM Karel Zak [off-list ref] wrote:

quoted

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.

The notification also reports new mount nodes, so we have no mount ID
<-> mount path mapping in userspace yet.

The another problem is that open(path) cannot be used if you have multiple
filesystems on the same mount point -- in this case (at least in theory)
you can get ID for by-path inaccessible filesystem.

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);

Yes, something like this is necessary. You do not want to depend
on paths if you want to read information about mountpoints.

 Karel

-- 
 Karel Zak  [off-list ref]
 http://karelzak.blogspot.com