Subject: Re: [PATCH RFC 2/6] mm: PGTABLE_HAS_P[MU]D_LEAVES config options

From: Peter Xu <[email-masked]>
Date: Thu, 22 Aug 2024 15:16:55 -0400

On Thu, Aug 22, 2024 at 05:22:03PM +0000, LEROY Christophe wrote:
> 
> 
> Le 18/07/2024 à 00:02, Peter Xu a écrit :
> > Introduce two more sub-options for PGTABLE_HAS_HUGE_LEAVES:
> > 
> >    - PGTABLE_HAS_PMD_LEAVES: set when there can be PMD mappings
> >    - PGTABLE_HAS_PUD_LEAVES: set when there can be PUD mappings
> > 
> > It will help to identify whether the current build may only want PMD
> > helpers but not PUD ones, as these sub-options will also check against the
> > arch support over HAVE_ARCH_TRANSPARENT_HUGEPAGE[_PUD].
> > 
> > Note that having them depend on HAVE_ARCH_TRANSPARENT_HUGEPAGE[_PUD] is
> > still some intermediate step.  The best way is to have an option say
> > "whether arch XXX supports PMD/PUD mappings" and so on.  However let's
> > leave that for later as that's the easy part.  So far, we use these options
> > to stably detect per-arch huge mapping support.
> > 
> > Signed-off-by: Peter Xu <[email-masked]>
> > ---
> >   include/linux/huge_mm.h | 10 +++++++---
> >   mm/Kconfig              |  6 ++++++
> >   2 files changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 711632df7edf..37482c8445d1 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -96,14 +96,18 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
> >   #define thp_vma_allowable_order(vma, vm_flags, tva_flags, order) \
> >   	(!!thp_vma_allowable_orders(vma, vm_flags, tva_flags, BIT(order)))
> >   
> > -#ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
> > -#define HPAGE_PMD_SHIFT PMD_SHIFT
> > +#ifdef CONFIG_PGTABLE_HAS_PUD_LEAVES
> >   #define HPAGE_PUD_SHIFT PUD_SHIFT
> >   #else
> > -#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> >   #define HPAGE_PUD_SHIFT ({ BUILD_BUG(); 0; })
> >   #endif
> >   
> > +#ifdef CONFIG_PGTABLE_HAS_PMD_LEAVES
> > +#define HPAGE_PMD_SHIFT PMD_SHIFT
> > +#else
> > +#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> > +#endif
> > +
> >   #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
> >   #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
> >   #define HPAGE_PMD_MASK	(~(HPAGE_PMD_SIZE - 1))
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 60796402850e..2dbdc088dee8 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -860,6 +860,12 @@ endif # TRANSPARENT_HUGEPAGE
> >   config PGTABLE_HAS_HUGE_LEAVES
> >   	def_bool TRANSPARENT_HUGEPAGE || HUGETLB_PAGE
> >   
> > +config PGTABLE_HAS_PMD_LEAVES
> > +	def_bool HAVE_ARCH_TRANSPARENT_HUGEPAGE && PGTABLE_HAS_HUGE_LEAVES
> > +
> > +config PGTABLE_HAS_PUD_LEAVES
> > +	def_bool HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD && PGTABLE_HAS_HUGE_LEAVES
> > +
> 
> What if an architecture has hugepages at PMD and/or PUD level and 
> doesn't support THP ?

What's the arch to be discussed here?

The whole purpose of this series so far is trying to make some pmd/pud
helpers that only defined with CONFIG_THP=on to be available even if not.
It means this series alone (or any future plan) shouldn't affect any arch
that has CONFIG_THP=off always.

But logically I think we should need some config option just to say "this
arch supports pmd mappings" indeed, even if CONFIG_THP=off.  When that's
there, we should perhaps add that option into this equation so
PGTABLE_HAS_*_LEAVES will also be selected in that case.

-- 
Peter Xu