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Mon, 20 Oct 2025 20:54:13 -0400 (EDT) Message-ID: <086bb120-22eb-43ff-a486-14e8eeb7dd80@maowtm.org> Date: Tue, 21 Oct 2025 01:54:11 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: 9P change breaks bpftrace running in qemu+9p? To: Song Liu , Dominique Martinet Cc: v9fs@lists.linux.dev, Eric Van Hensbergen , Latchesar Ionkov , Christian Schoenebeck , =?UTF-8?Q?Micka=C3=ABl_Sala=C3=BCn?= , bpf References: Content-Language: en-US From: Tingmao Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/20/25 22:52, Song Liu wrote: > Hi Dominique, > > On Mon, Oct 20, 2025 at 2:32 PM Dominique Martinet > 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