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

[Muchun Song]

> On Jan 16, 2026, at 01:23, Kiryl Shutsemau <kas@xxxxxxxxxx> 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.

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.

> 
> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov