Re: [PATCH v3 10/10] selftests/liveupdate: Add a test for preserving tmpfs files

[Pasha Tatashin]

On 11-04 17:59, Pratyush Yadav wrote:
> On Tue, Nov 03 2026, David Matlack wrote:
> 
> > Add luo_tmpfs, a kexec test for preserving tmpfs files and memfds, and
> > for retrieving tmpfs files onto a tmpfs mount with struct
> > liveupdate_shmem_retrieve_arg.
> >
> > Stage 1 preserves the source files of each test case. Stage 2 runs
> > each test case in its own directory, with its own tmpfs mounts, so that
> > block usage can be checked exactly with statfs() and st_blocks. The test
> > cases cover:
> >
> >  - The restored file: contents, size, position, seals and holes, for
> >    linked and unlinked source files.
> >
> >  - Creating the file: it is created like with open(O_TMPFILE), with the
> >    requested mode minus the umask, owned by the caller, and not linked
> >    until userspace links it, unless it asked for O_EXCL. Also O_RDONLY
> >    and O_WRONLY files, other open() flags such as O_APPEND, O_NONBLOCK
> >    and O_CLOEXEC, read-only mounts, and directories that the caller
> >    can't write to.
> >
> >  - Preserving files that can't be written to: a binary while it is
> >    executed, through a read-only fd, files on read-only mounts and
> >    superblocks, and immutable files, including sparse ones, whose holes
> >    are filled. Preserving someone else's file that the caller can't
> >    write to fails, and so does preserving a file through a write-only
> >    fd or an append-only file.
> >
> >  - Executing restored binaries: a restored binary can be executed
> >    before the session is finished if it was retrieved with O_RDONLY, but
> >    not if it was retrieved for writing, since LUO keeps the file open
> >    until then. These test cases execute copies of the test binary.
> >
> >  - Block accounting: the file fits a mount of exactly its size, and a
> >    large folio that extends past the end of the file is charged in full.
> >    Running out of space, or hitting the memcg limit, fails permanently,
> >    and leaves nothing charged to the mount or the memcg. The session can
> >    still be finished.
> >
> >  - memfds: they are only retrieved as memfds, with their seals, and
> >    files from tmpfs mounts only onto a tmpfs mount.
> >
> >  - Interactions with LUO: invalid arguments, which can be retried,
> >    retrieving a token twice, and outgoing sessions.
> >
> > Also check that the size of a preserved file is frozen, and that the
> > test doesn't trigger kernel warnings.
> >
> > Enable CONFIG_TRANSPARENT_HUGEPAGE for the large folio test cases,
> > CONFIG_MEMCG for the memcg test case and CONFIG_TMPFS_XATTR for
> > FS_IOC_SETFLAGS. Those test cases are skipped without them.
> >
> > tmpfs quotas are not tested, since the test runs as root, which is not
> > subject to quota limits.
> >
> > Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> > ---
> >  tools/testing/selftests/liveupdate/Makefile   |    1 +
> >  tools/testing/selftests/liveupdate/config     |    4 +
> >  .../testing/selftests/liveupdate/luo_tmpfs.c  | 2055 +++++++++++++++++
> 
> Umm, no way I am reading all this...
> 
> LLMs are great at generating tests, and I'm not saying there is no value
> in this. It is just that this massive code dump is unreviewable by
> humans. It is also unfair to the human reviewer because it took many
> orders of magnitude less effort to _write_ them with an LLM.
> 
> What do you all reckon we should do about this? Trust the LLM to get the
> testing right? Do we give up human maintenance of tests entirely? Do we
> do fault injection to test the tests themselves?
> 
> Or do we make the tests smaller and more human reviewable?

We need good test coverage, but I believe we can break this patch down 
into smaller changes where tested features are incrementally added.

> 
> >  .../selftests/liveupdate/run-vmtests.sh       |    1 +
> >  4 files changed, 2061 insertions(+)
> >  create mode 100644 tools/testing/selftests/liveupdate/luo_tmpfs.c
> >
> [...]
> 
> -- 
> Regards,
> Pratyush Yadav

-- 
Pasha