Re: [PATCH v3 3/3] kselftest: mm: introduce size_to_shift() helper

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Sep 14, 2026 at 04:08:30PM +0200, David Hildenbrand (Arm) wrote:
> On 9/14/26 16:06, Yeoreum Yun wrote:
> > On Mon, Sep 14, 2026 at 04:01:02PM +0200, David Hildenbrand (Arm) wrote:
> >> On 9/14/26 09:30, Yeoreum Yun wrote:
> >>> Introduce size_to_shift() for future users that need to
> >>> obtain shift of a huge-page which is other than pmd_psize().
> >>>
> >>> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
> >>> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> >>> ---
> >>>  tools/testing/selftests/mm/vm_util.c | 10 +++++++++-
> >>>  tools/testing/selftests/mm/vm_util.h |  1 +
> >>>  2 files changed, 10 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> >>> index fd1432dc6152..1fef1266429f 100644
> >>> --- a/tools/testing/selftests/mm/vm_util.c
> >>> +++ b/tools/testing/selftests/mm/vm_util.c
> >>> @@ -157,6 +157,14 @@ bool check_for_pattern(FILE *fp, const char *pattern, char *buf, size_t len)
> >>>  	return false;
> >>>  }
> >>>

Probably worth a comment here saying it's only for power-of-two sizes.

> >>> +uint64_t size_to_shift(uint64_t size)
> >>> +{
> >>> +	if (__builtin_popcountll(size) != 1)

I wonder if it wouldn't be better to just do the old trick of:

	if (!size || (size & (size - 1)))
		return 0;

?

> >>> +		return 0;
> >>> +
> >>> +	return ffsl(size) - 1;
> >>> +}
> >>
> >> Could likely just be an inline helper?
> >
> > Okay. If there is no comments on today, I'll post tomorrow with this.
> >
>
> No need to rush, leave some more days for people to catch up.

Agree with David seems better as an inline thing, the __builtin_xxx() should
be... built-in hopefully :P though I suggest something that wouldn't need that.

As long as strings.h is included in the header ofc for ffsl().

With the stuff above addressed, seems fine to me so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo