Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages

[David Hildenbrand (Red Hat)]

On 1/15/26 18:23, Kiryl Shutsemau wrote:
> On Thu, Jan 15, 2026 at 05:49:43PM +0100, David Hildenbrand (Red Hat) wrote:
> > On 1/15/26 15:45, Kiryl Shutsemau wrote:
> > > HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most
> > > vmemmap pages for huge pages and remapping the freed range to a single
> > > page containing the struct page metadata.
> > >
> > > With the new mask-based compound_info encoding (for power-of-2 struct
> > > page sizes), all tail pages of the same order are now identical
> > > regardless of which compound page they belong to. This means the tail
> > > pages can be truly shared without fake heads.
> > >
> > > Allocate a single page of initialized tail struct pages per NUMA node
> > > per order in the vmemmap_tails[] array in pglist_data. All huge pages
> > > of that order on the node share this tail page, mapped read-only into
> > > their vmemmap. The head page remains unique per huge page.
> > >
> > > This eliminates fake heads while maintaining the same memory savings,
> > > and simplifies compound_head() by removing fake head detection.
> > >
> > > Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> > > ---
> > >    include/linux/mmzone.h | 16 ++++++++++++++-
> > >    mm/hugetlb_vmemmap.c   | 44 ++++++++++++++++++++++++++++++++++++++++--
> > >    mm/sparse-vmemmap.c    | 44 ++++++++++++++++++++++++++++++++++--------
> > >    3 files changed, 93 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index 322ed4c42cfc..2ee3eb610291 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -82,7 +82,11 @@
> > >     * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect
> > >     * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit.
> > >     */
> > > -#define MAX_FOLIO_ORDER		get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G)
> > > +#ifdef CONFIG_64BIT
> > > +#define MAX_FOLIO_ORDER		(34 - PAGE_SHIFT)
> > > +#else
> > > +#define MAX_FOLIO_ORDER		(30 - PAGE_SHIFT)
> > > +#endif
> >
> > Where do these magic values stem from, and how do they related to the
> > comment above that clearly spells out 16G vs. 1G ?
>
> This doesn't change the resulting value: 1UL << 34 is 16GiB, 1UL << 30
> is 1G. Subtract PAGE_SHIFT to get the order.
>
> The change allows the value to be used to define NR_VMEMMAP_TAILS which
> is used specify size of vmemmap_tails array.

get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) should evaluate to 
a constant by the compiler.

See __builtin_constant_p handling in get_order().

If that is not working then we have to figure out why.

Was this only a specific config in where you ran into compile-time problems?

--
Cheers

David