Re: [PATCH v4 1/2] huge_mm: add stubs for THP-disabled configs

[hev]

Hi David,

On Thu, Mar 12, 2026 at 11:57 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 3/12/26 16:53, David Hildenbrand (Arm) wrote:
> > On 3/10/26 04:11, WANG Rui wrote:
> >> hugepage_global_enabled() and hugepage_global_always() only exist
> >> when CONFIG_TRANSPARENT_HUGEPAGE is set.  Add inline stubs that
> >> return false to let code compile when THP is disabled.
> >>
> >> Signed-off-by: WANG Rui <r@xxxxxx>
> >> ---
> >>  include/linux/huge_mm.h | 10 ++++++++++
> >>  1 file changed, 10 insertions(+)
> >>
> >> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> >> index a4d9f964dfde..badeebd4ea98 100644
> >> --- a/include/linux/huge_mm.h
> >> +++ b/include/linux/huge_mm.h
> >> @@ -570,6 +570,16 @@ void map_anon_folio_pmd_nopf(struct folio *folio, pmd_t *pmd,
> >>
> >>  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> >>
> >> +static inline bool hugepage_global_enabled(void)
> >> +{
> >> +    return false;
> >> +}
> >> +
> >> +static inline bool hugepage_global_always(void)
> >> +{
> >> +    return false;
> >> +}
> >> +
> >>  static inline bool folio_test_pmd_mappable(struct folio *folio)
> >>  {
> >>      return false;
> >
> > There are other ways to enable PMD THP. So I don't quite think this is
> > the right tool for the job.
>
> Ah, you care about file THPs ... gah.
>
> Why can't we simply do the alignment without considering the current
> setting?

The main motivation of raising the alignment here is to increase the
chance of getting PMD-sized THPs for executable mappings.

If THP is not in "always" mode, the kernel will not automatically
collapse file-backed mappings into THPs, so the increased alignment
would not actually improve THP usage.

In that case we would only be introducing additional padding in the
virtual address layout, which slightly reduces ASLR entropy without
providing a practical benefit.

That's why the current code limits this to the "always" mode.

Thanks,
Rui