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

[Muchun Song <muchun.song@linux.dev>]

> 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.

> 
> I think getting the array compile-time is the best shot.
> 
> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov