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

[Kiryl Shutsemau <kas@kernel.org>]

On Sat, Jan 17, 2026 at 10:38:48AM +0800, Muchun Song wrote:
> 
> 
> > On Jan 16, 2026, at 23:52, Kiryl Shutsemau <kas@kernel.org> wrote:
> > 
> > On Fri, Jan 16, 2026 at 10:38:02AM +0800, Muchun Song wrote:
> >> 
> >> 
> >>> On Jan 16, 2026, at 01:23, Kiryl Shutsemau <kas@kernel.org> 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@kernel.org>
> >>>>> ---
> >>>>> 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.
> >> 
> >> How about allocate ->vmemmap_tails array dynamically? If sizeof of struct
> >> page is not power of two, then we could optimize away this array. Besides,
> >> the original MAX_FOLIO_ORDER could work as well.
> > 
> > This is tricky.
> > 
> > We need vmemmap_tails array to be around early, in
> > hugetlb_vmemmap_init_early(). By the time, we don't have slab
> > functional yet.
> 
> I mean zero-size array at the end of pg_data_t, no slab is needed.

For !NUMA, the struct is in BSS. See contig_page_data.

Dynamic array won't fly there.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov