Re: [PATCH v2] ext4: improve str2hashbuf by processing 4-byte chunks and removing function pointers

[Guan-Chun Wu]

On Thu, Apr 09, 2026 at 10:10:50AM -0400, Theodore Tso wrote:
> On Sat, Nov 22, 2025 at 12:39:29PM +0800, Guan-Chun Wu wrote:
> > The original byte-by-byte implementation with modulo checks is less
> > efficient. Refactor str2hashbuf_unsigned() and str2hashbuf_signed()
> > to process input in explicit 4-byte chunks instead of using a
> > modulus-based loop to emit words byte by byte.
> > 
> > Additionally, the use of function pointers for selecting the appropriate
> > str2hashbuf implementation has been removed. Instead, the functions are
> > directly invoked based on the hash type, eliminating the overhead of
> > dynamic function calls.
> > 
> > Performance test (x86_64, Intel Core i7-10700 @ 2.90GHz, average over 10000
> > runs, using kernel module for testing):
> > 
> >     len | orig_s | new_s | orig_u | new_u
> >     ----+--------+-------+--------+-------
> >       1 |   70   |   71  |   63   |   63
> >       8 |   68   |   64  |   64   |   62
> >      32 |   75   |   70  |   75   |   63
> >      64 |   96   |   71  |  100   |   68
> >     255 |  192   |  108  |  187   |   84
> > 
> > This change improves performance, especially for larger input sizes.
> > 
> > Signed-off-by: Guan-Chun Wu <409411716@xxxxxxxxxxxxxx>
> 
> Apologies for the delay in looking at this.  It fell through the
> cracks on my end.
> 
> Because of how I'm a bit late with reviewing patches before the merge
> window, I'm going to be very conservative in which patches I'm going
> to land.  So this is going to be deferred until the next cycle, but I
> wanted to let you know that I haven't forgotten about it.
> 
> If this was a comprehensive set of Kunit tests for fs/ext4/hash.c, I
> might have taken it.  And that's something that I would look at adding
> for the next cycle, but if you'd be interested in creating the kunit
> tests for hash.c, that would be great.
> 
> 						- Ted

Thanks for the update.

I'd be happy to add Kunit tests for fs/ext4/hash.c. I'll work on them and
send a v3 patchset with the tests and the optimization in the next cycle.

Best regards,
Guan-Chun