Re: [PATCH v3 20/20] ext4: simplify calculation of blkoff in ext4_mb_new_blocks_simple

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

On Thu, Mar 16, 2023 at 06:19:40PM +0800, Kemeng Shi wrote:
> Hi Theodore, thanks for feedback. I will submit another patchset for
> mballoc and I would like to include this fix if no one else does. As
> new patches may be conflicted with old ones I submited, I would submit
> the new patchset after the old ones are fully reviewed and applied
> if this fix is not in rush. Thanks!

Hi, I've already taken the your patches into the dev branch; were
there any changes you were intending to make to your patches?

If you could submit a separate fix for the bug that I noticed, that
would be great.

Also, if you are interested in doing some more work in mballoc.c, I
was wondering if you would be interested in adding some Kunit tests
for mballoc.c.  A simple example Kunit test for ext4 can be found in
fs/ext4/inode_test.c.  (The convention is to place tests for foo.c in
foo_test.c.)

[1] https://docs.kernel.org/dev-tools/kunit/

In order to add mballoc Kunit tests, we will need to add some "mock"[2]
functions to simulate what happens when mballoc.c tries reading a
block bitmap.  My thinking was to have a test provide an array of some
data structure like this:

struct test_bitmap {
       unsigned int	start;
       unsigned int	len;
};

[2] https://en.wikipedia.org/wiki/Mock_object

... which indicates the starting block, and the length of a run of
blocks that are marked as in use, where the list of blocks are sorted
by starting block number, and where a starting block of ~0 indicates
the end of the list of block extents.

We would also need have a set of utility ext4 Kunit functions to
create "fake" ext4 superblocks and ext4_sb_info structures.

I was originally thinking that obvious starting Kunit tests would be
for fs/ext4/hash.c and fs/ext4/extents_status.c, since they require
the little or no "mocking" support.  However, there are so many
changes in fs/ext4/mballoc.c, the urgency in having unit tests for it
is getting more urgent --- since if there is a bug in one of these
functions, such as the one that I noted in
ext4_mb_new_blocks_simple(), since it's harder to exhaustively test
some of these smaller sub-functions in integration tests such as those
found in xfstests.  Unit tests are the best way to make sure we're
testing all of the code paths in a complex module such as mballoc.c

Cheers,

						- Ted