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

[David Hildenbrand <david@redhat.com>]

On 12.03.25 09:34, Brendan Jackman wrote:
> 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.

Ah okay, that makes sense.

> 
> 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'm afraid yes. Although allocating temp files form 9pfs is rather ... 
weird. :) One would assume that /tmp is usually backed by tmpfs. But 
well, a disto can do what it wants.

> 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.
> 

Yeah, we're trying ...

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

I assume it's broken in various ways to mimic that you are a file system 
which you are not.

Your approach is likely the easiest approach to deal with this 9pfs crap.

Can you document in the code+description better what we learned, and why 
we cannot even trust f_type with crappy 9pfs?

---
Cheers,

David / dhildenb