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Date: Thu, 30 May 2024 21:16:32 +0200
From: Emanuele Rocca <ema@debian.org>
To: David Howells <dhowells@redhat.com>
Cc: Andrea Righi <andrea.righi@canonical.com>,
	Jeff Layton <jlayton@kernel.org>, Steve French <smfrench@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Paulo Alcantara <pc@manguebit.com>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Eric Van Hensbergen <ericvh@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	linux-afs@lists.infradead.org, linux-cifs@vger.kernel.org,
	v9fs@lists.linux.dev, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Latchesar Ionkov <lucho@ionkov.net>,
	Christian Schoenebeck <linux_oss@crudebyte.com>,
	Luca Boccassi <bluca@debian.org>
Subject: Re: [PATCH v5 40/40] 9p: Use netfslib read/write_iter
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 <20231221132400.1601991-1-dhowells@redhat.com>
 <20231221132400.1601991-41-dhowells@redhat.com>
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Hi David,

On 2024-05-23 08:44, David Howells wrote:
> In https://bugs.launchpad.net/ubuntu/+source/autopkgtest/+bug/2056461 you say:
> 
> | It seems that kernel 6.8 introduced a regression in the 9pfs related to
> | caching and netfslib, that can cause some user-space apps to read content
> | from files that is not up-to-date (when they are used in a producer/consumer
> | fashion).
> 
> Can you clarify how these files are being used?

I don't know the details of the 9pfs operations involved, but still I
wanted to mention that to reliably reproduce the issue on a Debian
system one can run:

 autopkgtest-build-qemu unstable /tmp/sid.img
 autopkgtest -ddd -B dpdk -- autopkgtest-virt-qemu --debug --show-boot /tmp/sid.img

If the kernel installed in the guest VM is affected by this problem,
after a while the test hangs with something like:

 autopkgtest-virt-qemu: DBG: executing copydown /tmp/alog/tests-tree/ /tmp/autopkgtest.uG6tsJ/build.6QA/src/
 [...]
 autopkgtest-virt-qemu: DBG:  +>?

Full logs at https://people.debian.org/~ema/1072004-6.10-rc1.log

Part of the code mounting the 9pfs in case it helps is at:
https://salsa.debian.org/ci-team/autopkgtest/-/blob/master/virt/autopkgtest-virt-qemu#L290

I could reproduce the issue with both 6.9.2 and 6.10-rc1.