Re: [PATCH] ovl: add ioctls to retrieve layer file descriptors

On Wed, Jul 8, 2026, at 3:01 PM, Amir Goldstein wrote:

> If you want to keep a pool of mounted erofs images, you could do that
> in userspace -
> create a service that indexes mounted erofs images by unique mount poi=
nt paths.

You're right that it=E2=80=99d be possible to do this in user space.=20

However, overlayfs is used by many things (it=E2=80=99s a powerful =E2=80=
=9CSwiss army knife=E2=80=9D!) and this ability to reliably introspect i=
ts components is I think generally useful.

For example: https://github.com/systemd/systemd/issues/35017#issuecommen=
t-2457333218

Various tools want to know the backing filesystem(s) and/or block device=
s thereof, and if we don=E2=80=99t have something like this in the gener=
al case every overlayfs user would need to agree on a scheme to store th=
is data out of band - and manage its lifecycle as mounts change and deal=
 with where that data is stored with respect to mount namespaces etc.

Even if we had this API, would it be better to have a user space cache s=
erver for the original use case here? Maybe. I could personally go eithe=
r way.

But the introspection I think really would be generally useful and there=
 are code bases (in systemd and in composefs at least) that would start =
using it (with fallback for old kernels) for at least that use case righ=
t away.