Re: [PATCH v5 10/17] mm/hugetlb: switch HugeTLB to section-based vmemmap optimization

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

> On Aug 25, 2026, at 21:12, Qi Zheng <qi.zheng@linux.dev> wrote:
> 
> 
> 
> On 8/25/26 4:46 PM, Muchun Song wrote:
>> HugeTLB bootmem vmemmap optimization still carries its own early setup
>> path, including pre-populating optimized mappings before the generic
>> sparse-vmemmap code runs.
>> Now that the section-based vmemmap optimization can derive HugeTLB
>> vmemmap deduplication from section metadata, HugeTLB only needs to mark
>> the bootmem huge page range with the appropriate order. The generic
>> sparse-vmemmap population path can then allocate and map the shared tail
>> vmemmap pages without any HugeTLB-specific early population code.
>> Do that by setting the section order when a bootmem huge page is
>> allocated and dropping the dedicated pre-HVO helpers and related
>> special-casing.
>> This removes duplicate early setup logic and switches HugeTLB to the
>> section-based vmemmap optimization path.
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>> ---
>> v3:
>> - Use the order-based helper for the bootmem vmemmap-optimized check
>> v2:
>> - Collect Acked-by from Mike Rapoport
>> ---
>>  include/linux/hugetlb.h |  1 -
>>  include/linux/mm.h      |  3 --
>>  mm/hugetlb.c            | 30 ++------------
>>  mm/hugetlb_vmemmap.c    | 90 +++--------------------------------------
>>  mm/hugetlb_vmemmap.h    | 14 +++----
>>  mm/sparse-vmemmap.c     | 31 --------------
>>  mm/sparse.h             | 27 +++++++++++++
>>  7 files changed, 42 insertions(+), 154 deletions(-)
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 16c4c4caa126..fe28f98e1b22 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -171,7 +171,6 @@ struct address_space *hugetlb_folio_mapping_lock_write(struct folio *folio);
>>    extern int movable_gigantic_pages __read_mostly;
>>  extern int sysctl_hugetlb_shm_group __read_mostly;
>> -extern struct list_head huge_boot_pages[MAX_NUMNODES];
>>    void hugetlb_bootmem_struct_page_init(void);
>>  void hugetlb_bootmem_alloc(void);
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index dd09c438fa23..441bd39eab73 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -5159,9 +5159,6 @@ int vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>          int node, struct vmem_altmap *altmap);
>>  int vmemmap_populate(unsigned long start, unsigned long end, int node,
>>   struct vmem_altmap *altmap);
>> -int vmemmap_populate_hvo(unsigned long start, unsigned long end,
>> -  unsigned int order, struct zone *zone,
>> -  unsigned long headsize);
>>  void vmemmap_wrprotect_hvo(unsigned long start, unsigned long end, int node,
>>     unsigned long headsize);
>>  void vmemmap_populate_print_last(void);
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 04e6c4244cd6..fbb0c83bea79 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -52,6 +52,7 @@
>>  #include "hugetlb_cma.h"
>>  #include "hugetlb_internal.h"
>>  #include "mm_init.h"
>> +#include "sparse.h"
>>  #include <linux/page-isolation.h>
>>    int hugetlb_max_hstate __read_mostly;
>> @@ -59,7 +60,7 @@ unsigned int default_hstate_idx;
>>  struct hstate hstates[HUGE_MAX_HSTATE];
>>    __initdata nodemask_t hugetlb_bootmem_nodes;
>> -__initdata struct list_head huge_boot_pages[MAX_NUMNODES];
>> +static struct list_head huge_boot_pages[MAX_NUMNODES] __initdata;
>> /*
>>  * Due to ordering constraints across the init code for various
>> @@ -3139,6 +3140,7 @@ static bool __init alloc_bootmem_huge_page(struct hstate *h, int nid)
>>   	} else {
>>   		list_add_tail(&m->list, &huge_boot_pages[nid]);
>>   		m->flags |= HUGE_BOOTMEM_ZONES_VALID;
>> + 		hugetlb_vmemmap_optimize_bootmem_page(m);
>>   		/*
>>   		 * Only initialize the head struct page in memmap_init_reserved_pages,
>>   		 * rest of the struct pages will be initialized by the HugeTLB
>> @@ -3299,6 +3301,7 @@ static void __init gather_bootmem_prealloc_node(unsigned long nid)
>>   	 * this folio.
>>   	 */
>>   	folio_set_hugetlb_vmemmap_optimized(folio);
>> + 	section_set_order_range(folio_pfn(folio), folio_nr_pages(folio), 0);
> 
> So section->order is only used during initialization. Is it ever
> accessed later at runtime? If not, can we just skip zeroing it out?

You have keenly noticed a detail. This was actually done deliberately,
because in patch 14, HUGE_BOOTMEM_HVO was removed and replaced with 
section->order. The ->order field may store a value that is less than
VMEMMAP_OPTIMIZATION_MIN_ORDER, so clearing it to 0 here is to prevent
potential issues with this memory region during the hotplug/hotremove
process in the future (in my future series).

However, is there also a way to avoid clearing it to 0? There is. We
could add an extra check in hugetlb_vmemmap_optimize_bootmem_page to
only call section_set_order_range when the current hstate->order is
greater than or equal to VMEMMAP_OPTIMIZATION_MIN_ORDER. I just feel
that this would add a bit more code. And then during the boot phase,
doing one extra zeroing-out doesn't introduce much overhead anyway.
Therefore, I chose the simpler implementation approach.

Muchun,
Thanks.