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

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Message-ID: <aWpTopypeRTOIVGR@thinkstation>
Date: Fri, 16 Jan 2026 15:52:05 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: Muchun Song <muchun.song@...ux.dev>
Cc: "David Hildenbrand (Red Hat)" <david@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Usama Arif <usamaarif642@...il.com>, Frank van der Linden <fvdl@...gle.com>, 
	Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>, 
	Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Zi Yan <ziy@...dia.com>, Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>, 
	Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>, kernel-team@...a.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages

On Fri, Jan 16, 2026 at 10:38:02AM +0800, Muchun Song wrote:
> 
> 
> > On Jan 16, 2026, at 01:23, Kiryl Shutsemau <kas@...nel.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@...nel.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 think getting the array compile-time is the best shot.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov