Re: 9P change breaks bpftrace running in qemu+9p?

[Tingmao Wang <m@maowtm.org>]

On 10/20/25 22:52, Song Liu wrote:
> Hi Dominique,
> 
> On Mon, Oct 20, 2025 at 2:32 PM Dominique Martinet
> <asmadeus@codewreck.org> wrote:
>>
>> Song Liu wrote on Mon, Oct 20, 2025 at 12:40:23PM -0700:
>>> I am running qemu 9.2.0 and bpftrace v0.24.0. I don't think anything is
>>> very special here.
>>
>> I don't reproduce either (qemu 9.2.4 and bpftrace v0.24.1, I even went
>> and installed vmtest to make sure), trying both my branch and a pristine
>> v6.18-rc2 kernel -- what's the exact commit you're testing and could you
>> attach your .config ?
> 
> Attached, please find the config file.
> 
> I tried to debug this, and found that the issue disappears when I remove
> v9fs_lookup_revalidate from v9fs_dentry_operations. But I couldn't figure
> out why d_revalidate() is causing such an issue.

I've compiled qemu 9.2.0 and download the binary build of bpftrace v0.24.0
from GitHub [1], and compiled kernel with your config, but unfortunately I
still can't reproduce it...

I do now get this message sometimes (probably unrelated?):
bpftrace (148) used greatest stack depth: 11624 bytes left

I don't really know how to proceed right now but I will have it run in a
loop and see if I can hit it by chance.

If you can reproduce it frequently and can debug exactly what is returning
-EIO in v9fs_lookup_revalidate that would probably be very helpful, or if
you can enable 9p debug outputs and see what's happening around the time
of error (CONFIG_NET_9P_DEBUG=y and also debug=5 mount options - I'm not
sure how to get vmtest to use a custom mount option but if it's
reproducible in plain QEMU that's also an option) that might also be
informative I think?  I'm happy to take a deeper look (although I'm of
course less of an expert than Dominique so hopefully he can also give some
opinion).

I'm also curious if this can happen with just a usual `stat` or other
operations (not necessarily caused by dentry revalidation, and thus not
necessarily to do with my patch)

[1]: https://github.com/bpftrace/bpftrace/releases/tag/v0.24.0

> 
> Thanks,
> Song