[SRU][N][PATCH 0/1] fix autopkgtest-virt-qemu regression with 6.8

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/2056461

[Impact]

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).

It seems that the offending commit is this one:

 80105ed2fd27 ("9p: Use netfslib read/write_iter")

Reverting the commit seems to fix the problem. However the actual bug
might be in netfslib or how netfslib is used in the 9p context.

The regression has been reported upstream and we are still investigating
(https://lore.kernel.org/lkml/Zj0ErxVBE3DYT2Ea@gpd/).

In the meantime it probably makes sense to temporarily revert the commit
as a SAUCE patch. Then we will drop the SAUCE patch once we'll have a
proper fix upstream.

[Test case]

The following test should complete correctly without any timeout:

  pull-lp-source -d hello
  autopkgtest-buildvm-ubuntu-cloud -r noble
  autopkgtest -U hello*.dsc -- qemu ./autopkgtest-noble-amd64.img

[Fix]

Revert the following commit (until we have a proper fix upstream):

 80105ed2fd27 ("9p: Use netfslib read/write_iter")

[Regression potential]

We may experience other regressions related to 9pfs with this change,
however it's quite unlikely to happen since we are reverting a commit,
restoring the previous behavior.