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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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@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.

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