Re: [PATCH v5 02/12] mm/sparse-vmemmap: allocate shared tail page array dynamically

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

> On Sep 29, 2026, at 15:21, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> On 9/27/26 04:54, Muchun Song wrote:
>> Commit 622026e87c40 ("mm/hugetlb: remove fake head pages") added the
>> per-zone vmemmap_tails array. Its size depends on MAX_FOLIO_ORDER, which
>> had been moved to mmzone.h in preparation for the array.
>> 
>> PUD_ORDER is defined by linux/pgtable.h, which cannot be included from
>> mmzone.h without creating an include cycle. It was therefore open-coded
>> as PUD_SHIFT - PAGE_SHIFT.
>> 
>> This removed the dependency on PUD_ORDER, but not the underlying
>> dependency on architecture page-table definitions. PUD_SHIFT is
>> generally provided by architecture page-table headers, which are not
>> guaranteed to have been included when mmzone.h is parsed.
>> 
>> The dependency remained hidden because vmemmap_tails was originally
>> guarded by CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP. Under that condition,
>> MAX_FOLIO_ORDER resolves to either MAX_PAGE_ORDER or the fixed HugeTLB
>> limit, rather than the PUD_SHIFT-based definition.
>> 
>> Device DAX, however, does not require CONFIG_HUGETLB_PAGE. When it is
>> converted to use section-based vmemmap optimization, MAX_FOLIO_ORDER can
>> resolve to PUD_SHIFT - PAGE_SHIFT while it is being used to size
>> vmemmap_tails. This would make struct zone depend on architecture
>> page-table definitions being available when mmzone.h is parsed.
>> 
>> Replace the embedded array with a pointer and allocate it on first use.
>> This moves the order-count evaluation into sparse-vmemmap.c, after the
>> architecture page-table definitions are available, and removes the
>> dependency from mmzone.h.
>> 
>> Removing the compile-time array also removes the original reason for
>> keeping MAX_FOLIO_ORDER and the vmemmap optimization sizing definitions
>> in mmzone.h. Follow-up cleanups can place each definition in the header
>> owned by its respective subsystem.
>> 
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> v5:
>> - Add this patch to fix the RISC-V build failure under the configuration
>>  reported by the kernel test robot
>> ---
>> include/linux/mmzone.h |  7 +------
>> mm/sparse-vmemmap.c    | 35 +++++++++++++++++++++++++++++++----
>> 2 files changed, 32 insertions(+), 10 deletions(-)
>> 
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index acd94cecc0d3..68807ff7f946 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -113,11 +113,6 @@
>> (VMEMMAP_OPTIMIZATION_PAGES * PAGE_SIZE / sizeof(struct page))
>> #define VMEMMAP_OPTIMIZATION_MIN_ORDER (ilog2(VMEMMAP_OPTIMIZATION_NR_STRUCT_PAGES) + 1)
>> 
>> -#define __VMEMMAP_OPTIMIZATION_NR_ORDERS \
>> - 	(MAX_FOLIO_ORDER - VMEMMAP_OPTIMIZATION_MIN_ORDER + 1)
>> -#define VMEMMAP_OPTIMIZATION_NR_ORDERS \
>> - 	(__VMEMMAP_OPTIMIZATION_NR_ORDERS > 0 ? __VMEMMAP_OPTIMIZATION_NR_ORDERS : 0)
>> -
>> enum migratetype {
>> MIGRATE_UNMOVABLE,
>> MIGRATE_MOVABLE,
>> @@ -1156,7 +1151,7 @@ struct zone {
>> atomic_long_t vm_stat[NR_VM_ZONE_STAT_ITEMS];
>> atomic_long_t vm_numa_event[NR_VM_NUMA_EVENT_ITEMS];
>> #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>> - 	struct page *vmemmap_tails[VMEMMAP_OPTIMIZATION_NR_ORDERS];
>> + 	struct page **vmemmap_tails;
>> #endif
>> } ____cacheline_internodealigned_in_smp;
>> 
> 
> 
> [...]
> 
>> struct page __ref *vmemmap_shared_tail_page(unsigned int order, struct zone *zone)
>> {
>> 	void *addr;
>> - 	struct page *page;
>> + 	struct page *page, **pages;
>> 	const unsigned int idx = order - VMEMMAP_OPTIMIZATION_MIN_ORDER;
>> 
>> 	 (WARN_ON_ONCE(idx >= VMEMMAP_OPTIMIZATION_NR_ORDERS))
>> 		return NULL;
>> 
>> - 	page = READ_ONCE(zone->vmemmap_tails[idx]);
>> + 	pages = READ_ONCE(zone->vmemmap_tails) ? : vmemmap_tails_alloc(zone);
> 
> This reads much nicer if you handle the  READ_ONCE(zone->vmemmap_tails) inside
> the function.
> 
> pages = vmemmap_tails(zone);

Sounds good.

> 
> So just place the entire logic of obtaining the array in there.

No problem.

Thanks,
Muchun

> 
> 
> Apart from that LGTM.
> 
> -- 
> Cheers,
> 
> David