Re: [PATCH] ext4: fix crash on test_mb_mark_used kunit tests

["Theodore Ts'o" <tytso@mit.edu>]

On Fri, Jul 25, 2025 at 01:06:18PM +0200, Jan Kara wrote:
> > This patch applies to the kernel that has only merged bbe11dd13a3f
> > ("ext4: fix largest free orders lists corruption on mb_optimize_scan
> > switch"), but not merged 458bfb991155 ("ext4: convert free groups order
> > lists to xarrays").
> 
> Hum, I think it would be best to just squash this into bbe11dd13a3f and
> then just rebase & squash the other unittest fixup to the final commit when
> we have to rebase anyway. Because otherwise backports to stable kernel will
> quickly become rather messy.

What I ended up doing was to add a squashed combination of these two
commits and dropped it in before the block allocation scalabiltity
with the following commit description:

    ext4: initialize superblock fields in the kballoc-test.c kunit tests
    
    Various changes in the "ext4: better scalability for ext4 block
    allocation" patch series have resulted in kunit test failures, most
    notably in the test_new_blocks_simple and the test_mb_mark_used tests.
    The root cause of these failures is that various in-memory ext4 data
    structures were not getting initialized, and while previous versions
    of the functions exercised by the unit tests didn't use these
    structure members, this was arguably a test bug.
    
    Since one of the patches in the block allocation scalability patches
    is a fix which is has a cc:stable tag, this commit also has a
    cc:stable tag.
    
    CC: stable@vger.kernel.org
    Link: https://lore.kernel.org/r/20250714130327.1830534-1-libaokun1@huawei.com
    Link: https://patch.msgid.link/20250725021550.3177573-1-yi.zhang@huaweicloud.com
    Link: https://patch.msgid.link/20250725021654.3188798-1-yi.zhang@huaweicloud.com
    Reported-by: Guenter Roeck <linux@roeck-us.net>
    Closes: https://lore.kernel.org/linux-ext4/b0635ad0-7ebf-4152-a69b-58e7e87d5085@roeck-us.net/
    Tested-by: Guenter Roeck <linux@roeck-us.net>
    Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Then in the commit "ext4: convert free groups order lists to xarrays"
which removed list_head, I modified it to remove the linked list
initialization from mballoc-test.c, since that's the commit which
removed those structures.

In the future, we should try to make sure that when we modify data
structures to add or remove struct elements, that we also make sure
that kunit test should also be updated.  To that end, I've updated the
kbuild script[1] in xfstests-bld repo so that "kbuild --test" will run
the Kunit tests.  Hopefully reducing the friction for running tests
will encourage more kunit tests to be created and so they will kept
under regular maintenance.

[1] https://github.com/tytso/xfstests-bld/blob/master/kernel-build/kbuild

Cheers,

					- Ted