Re: [PATCH v3 08/10] selftests/mm: Skip gup_longerm tests on weird filesystems

[Brendan Jackman <jackmanb@google.com>]

On Tue, Mar 11, 2025 at 08:53:02PM +0100, David Hildenbrand wrote:
> > 2. 9pfs seems to pass the f_type through from the host. So you can't
> > detect it this way anyway.
> > 
> > [3. I guess overlayfs & friends would also be an issue here although
> > that doesn't affect my usecase.]
> > 
> > Anyway, I think we would have to scrape /proc/mounts to do this :(
> > 
> 
> The question I am asking myself: is this a 9pfs design bug or is it a 9pfs
> hypervisor bug. Because we shouldn't try too hard to work around hypervisor
> bugs.
> 
> Which 9pfs implementation are you using in the hypervisor?

I'm using QEMU via virtme-ng. IIUC virtme-ng knows how to use viortfs
for the rootfs, but for individually-mounted directories with
--rwdir/--rodir it uses 9pfs unconditionally.

Even if it's a bug in QEMU, I think it is worth working around this
one way or another. QEMU by far the most practical way to run these
tests, and virtme-ng is probably the most popular/practical way to do
that. I think even if we are confident it's just a bunch of broken
code that isn't even in Linux, it's pragmatic to spend a certain
amount of energy on having green tests there.

(Also, this f_type thing might be totally intentional specified
filesystem behaviour, I don't know).