Re: [PATCH v3 08/17] mm/sparse-vmemmap: support section-based vmemmap optimization

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

On 2026/8/6 11:04, Muchun Song wrote:
>
>
> On 2026/8/4 11:55, Muchun Song wrote:
>> Teach sparse-vmemmap population code to use the compound page order
>> when deciding whether a vmemmap page can be optimized.
>>
>> With this information, the common sparse-vmemmap population path can
>> allocate or reuse shared tail vmemmap pages directly instead of relying
>> on HugeTLB-specific handling.
>>
>> This centralizes vmemmap optimization logic in the sparse-vmemmap code,
>> based on section metadata, and prepares for sharing the same mechanism
>> across different users of vmemmap optimization, including HugeTLB and
>> DAX.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> v2:
>> - Keep vmemmap accounting and population logic in sparse-vmemmap.c
>>    (suggested by Mike Rapoport)
>> - Move vmemmap_get_tail() before its first use instead of adding only a
>>    forward declaration in the previous patch (suggested by Mike 
>> Rapoport)
>> - Simplify the PMD path handling for HVO-covered sections
>> ---
>>   mm/sparse-vmemmap.c | 36 ++++++++++++++++++++++++++++++------
>>   mm/sparse.c         |  4 ++--
>>   mm/sparse.h         |  7 +++++++
>>   3 files changed, 39 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index b770fe2428fd..b69a7af76858 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -186,6 +186,11 @@ static __meminit struct page 
>> *vmemmap_get_tail(unsigned int order, struct zone *
>>         return tail;
>>   }
>> +#else
>> +static inline struct page *vmemmap_get_tail(unsigned int order, 
>> struct zone *zone)
>> +{
>> +    return NULL;
>> +}
>>   #endif
>>     static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, 
>> unsigned long addr, int node,
>> @@ -193,12 +198,24 @@ static pte_t * __meminit 
>> vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
>>                          unsigned long ptpfn, unsigned long flags)
>>   {
>>       pte_t *pte = pte_offset_kernel(pmd, addr);
>> +    unsigned long pfn = page_to_pfn((struct page *)addr);
>> +
>>       if (pte_none(ptep_get(pte))) {
>>           pte_t entry;
>> -        void *p;
>> +
>> +        if (pfn_vmemmap_optimizable(pfn) && ptpfn == (unsigned 
>> long)-1) {
>> +            unsigned int order = pfn_to_section_order(pfn);
>> +            struct zone *zone = pfn_to_zone(pfn, node);
>> +            struct page *page = vmemmap_get_tail(order, zone);
>> +
>> +            if (!page)
>> +                return NULL;
>> +            ptpfn = page_to_pfn(page);
>> +        }
>>             if (ptpfn == (unsigned long)-1) {
>> -            p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>> +            void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>> +
>>               if (!p)
>>                   return NULL;
>>               ptpfn = PHYS_PFN(__pa(p));
>> @@ -217,7 +234,8 @@ static pte_t * __meminit 
>> vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
>>           }
>>           entry = pfn_pte(ptpfn, PAGE_KERNEL);
>>           set_pte_at(&init_mm, addr, pte, entry);
>> -    }
>> +    } else if (WARN_ON_ONCE(pfn_vmemmap_optimizable(pfn)))
>> +        return NULL;
>>       return pte;
>>   }
>>   @@ -406,6 +424,9 @@ int __meminit 
>> vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>       pmd_t *pmd;
>>         for (addr = start; addr < end; addr = next) {
>> +        unsigned long pfn = page_to_pfn((struct page *)addr);
>> +        const struct mem_section *ms = __pfn_to_section(pfn);
>> +
>>           next = pmd_addr_end(addr, end);
>>             pgd = vmemmap_pgd_populate(addr, node);
>> @@ -421,7 +442,7 @@ int __meminit vmemmap_populate_hugepages(unsigned 
>> long start, unsigned long end,
>>               return -ENOMEM;
>>             pmd = pmd_offset(pud, addr);
>> -        if (pmd_none(pmdp_get(pmd))) {
>> +        if (pmd_none(pmdp_get(pmd)) && 
>> !section_vmemmap_optimizable(ms)) {
>>               void *p;
>>                 p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
>> @@ -439,8 +460,11 @@ int __meminit 
>> vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>                    */
>>                   return -ENOMEM;
>>               }
>> -        } else if (vmemmap_check_pmd(pmd, node, addr, next))
>> +        } else if (vmemmap_check_pmd(pmd, node, addr, next)) {
>> +            if (WARN_ON_ONCE(section_vmemmap_optimizable(ms)))
>> +                return -ENOTSUPP;
>>               continue;
>> +        }
>>           if (vmemmap_populate_basepages(addr, next, node, altmap))
>>               return -ENOMEM;
>>       }
>> @@ -648,7 +672,7 @@ void offline_mem_sections(unsigned long 
>> start_pfn, unsigned long end_pfn)
>>       }
>>   }
>>   -static int __meminit section_nr_vmemmap_pages(unsigned long pfn, 
>> unsigned long nr_pages,
>> +int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned 
>> long nr_pages,
>
> The kernel test robot reported a compilation issue: when
> CONFIG_MEMORY_HOTPLUG = n && CONFIG_SPARSEMEM_VMEMMAP=y,
> section_nr_vmemmap_pages is undefined. This problem is easy to fix, and
> I will move the entire function outside the CONFIG_MEMORY_HOTPLUG guard
> in the next version. 

Hi,

I will wait a few more days to see if there are any reviews. If there is
no further input during this period, I will update a new version to resolve
this compilation issue.

Muchun,
Thanks