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

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Message-ID: <aWk1tZyFZOOkF0AH@thinkstation>
Date: Thu, 15 Jan 2026 18:58:39 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Muchun Song <muchun.song@...ux.dev>, 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 Thu, Jan 15, 2026 at 06:41:44PM +0100, David Hildenbrand (Red Hat) wrote:
> 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@...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.
> 
> 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.

asm-offsets.s compilation fails:

../include/linux/mmzone.h:1574:16: error: fields must have a constant size:
      'variable length array in structure' extension will never be supported
 1574 |         unsigned long vmemmap_tails[NR_VMEMMAP_TAILS];

Here's how preprocessor dump of vmemmap_tails looks like:

 unsigned long vmemmap_tails[(get_order(1 ? (0x400000000ULL) : 0x40000000) - (( __builtin_constant_p(2 * ((1UL) << 12) / sizeof(struct page)) ? ((2 * ((1UL) << 12) / sizeof(struct page)) < 2 ? 0 : 63 - __builtin_clzll(2 * ((1UL) << 12) / sizeof(struct page))) : (sizeof(2 * ((1UL) << 12) / sizeof(struct page)) <= 4) ? __ilog2_u32(2 * ((1UL) << 12) / sizeof(struct page)) : __ilog2_u64(2 * ((1UL) << 12) / sizeof(struct page)) )) + 1)];

And here's get_order():

static inline __attribute__((__gnu_inline__)) __attribute__((__unused__)) __attribute__((no_instrument_function)) __attribute__((__always_inline__)) __attribute__((__const__)) int get_order(unsigned long size)
{
 if (__builtin_constant_p(size)) {
  if (!size)
   return 64 - 12;

  if (size < (1UL << 12))
   return 0;

  return ( __builtin_constant_p((size) - 1) ? (((size) - 1) < 2 ? 0 : 63 - __builtin_clzll((size) - 1)) : (sizeof((size) - 1) <= 4) ? __ilog2_u32((size) - 1) : __ilog2_u64((size) - 1) ) - 12 + 1;
 }

 size--;
 size >>= 12;



 return fls64(size);

}

I am not sure why it is not compile-time constant. I have not dig
deeper.

Switching to ilog2(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) - PAGE_SHIFT works,
but I personally find my variant more readable.

Do you want me to dig deeper to check if making get_order() work
possible?

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

I am not aware about any particular config dependency. Seems to be
everywhere.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov