Re: Reading/changing projid of a symlink

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jun 12, 2018 at 03:17:27PM -0700, Ilya Pronin wrote:
> So the only way to remove a symlink from a project is to remove it?

Yes.

> Here's my use case. In Apache Mesos we use XFS project quotas for disk
> space isolation. Every container sandbox is allocated its own project
> ID. When the container terminates its project ID is unset from the
> sandbox and files inside it, and can be used for other containers.
> Sandboxes of terminated containers stay on the host for some time for
> debugging, etc. With such approach the inability to remove projid from
> symlinks presents a small problem: when a project ID is reused for a
> new container, any lingering symlinks that still have that project ID
> will contribute to disk usage of the new container. Typically not
> much, but still that's inaccurate. I guess we'll need to change when
> we "deallocate" projids.

There isn't really much we can do in XFS about it.  Traditionally in
all Unix variants you can never get a file descriptor for a symlink,
and all normal file operations always follow symlinks.  So you need
special new syscalls to handle the symlink.  Now Linux added the
O_PATH mount option a while ago which actually allows you to get
a file descriptor on symlinks, but just very limited ones that do
not actually support calls into the underlying file system.

So to support your use case we'd either need a new lioctl system
call, or some way to support limited ioctls on O_PATH fds.  Either
would be a VFS change and would turn into a huge discussion.  Once
that is done however wiring up the XFS bits would be trivial.