linux-kernel - Re: [PATCH] 9p/virtio: restrict page pinning to user_backed

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Message-ID: <aUSK8vrhPLAGdQlv@codewreck.org>
Date: Fri, 19 Dec 2025 08:14:58 +0900
From: asmadeus@...ewreck.org
To: Christian Schoenebeck <linux_oss@...debyte.com>
Cc: Eric Van Hensbergen <ericvh@...nel.org>,
	Latchesar Ionkov <lucho@...kov.net>,
	Chris Arges <carges@...udflare.com>, v9fs@...ts.linux.dev,
	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] 9p/virtio: restrict page pinning to user_backed_iter()
 iovec

Christian Schoenebeck wrote on Thu, Dec 18, 2025 at 04:14:45PM +0100:
> > Won't be the first time I can't reproduce, but what kind of workload are
> > you testing?
> > Anything that might help me try to reproduce (like VM cpu count/memory)
> > will be appreciated, corruptions are Bad...
> 
> Debian Trixie guest running as 9p rootfs in QEMU, 4 cores, 16 GB.
> 
> Compiling a bunch of projects with GCC works fine without errors, but with 
> clang it's very simple for me to reproduce. E.g. just a very short C++ file 
> that pulls in some system headers:
> 
> #include <utility>
> #include <sys/cdefs.h>
> #include <limits>
> 
> Then running 3 times: clang++ -c foo.cpp -std=c++17
> 
> The first 2 clang runs succeed, the 3rd clang run then always blows up for 
> anything else than cache=none, various spurious clang errors on those system 
> headers like

Thanks, I can't reproduce with this example, but building linux with
`make LLVM=1` does blow up on debian... even with cache=none actually?

I couldn't reproduce running the same rootfs directory in a container so
I don't think I corrupted my image, it appears to be reading junk? short
reads perhaps?...
(Interestingly, it doesn't seem to blow up on an alpine rootfs, I wonder
what's different...)

I'm now getting late for work but at least there's something I can
reproduce, I'll have a closer look ASAP, thank you.

-- 
Dominique Martinet | Asmadeus