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

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

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@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.
> 
> 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?



Some history on the SZ change here: https://lore.kernel.org/all/a31e6d70-9275-4277-991b-9de1aea03cd7@csgroup.eu/

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

Using SZ_16G/SZ_1G, is self-documenting. I'm fine with repeating the ilog2 like:


ifdef CONFIG_64BIT
#define MAX_FOLIO_ORDER		(ilog2(SZ_16G) - PAGE_SHIFT)
...


Also, make sure to spell that out in the patch description.


Figuring out why we don't get a constant would be even nicer ... or why this does something else
than expected.

-- 
Cheers

David