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

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Message-ID: <15a0dd88-3ec5-4538-ae6c-d2ad39f284e1@kernel.org>
Date: Thu, 15 Jan 2026 20:46:02 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...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 1/15/26 20:33, David Hildenbrand (Red Hat) wrote:
> On 1/15/26 19:58, Kiryl Shutsemau wrote:
>> 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.
> 
> Very weird. Almost sounds like a bug given that get_order() ends up using ilog2.
> 
> But it gets even weirder:
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6f959d8ca4b42..a54445682ccc4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2281,6 +2281,9 @@ static inline unsigned long folio_nr_pages(const struct folio *folio)
>     * 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)
> +
> +static_assert(__builtin_constant_p(MAX_FOLIO_ORDER));
> +
>    #else
>    /*
>     * Without hugetlb, gigantic folios that are bigger than a single PUD are
> 
> gives me
> 
> 
> ./include/linux/build_bug.h:78:41: error: static assertion failed: "__builtin_constant_p(MAX_FOLIO_ORDER)"
>      78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>         |                                         ^~~~~~~~~~~~~~
> ./include/linux/build_bug.h:77:34: note: in expansion of macro '__static_assert'
>      77 | #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
>         |                                  ^~~~~~~~~~~~~~~
> ./include/linux/mm.h:2285:1: note: in expansion of macro 'static_assert'
>    2285 | static_assert(__builtin_constant_p(MAX_FOLIO_ORDER));
>         | ^~~~~~~~~~~~~
> 
> And reversing the condition fixes it.
> 
> ... so it is a constant? Huh?

I've been staring at the computer for too long, this is not BUILD_BUG 
semantics. So we don't get a constant.

For some reason :)

Even when I just use get_order(4096).

-- 
Cheers

David