Re: [PATCH 3/3] mm: huge_memory: use prot_numa_skip() for pmd folio

[David Hildenbrand <david@redhat.com>]

On 14.10.25 08:10, Kefeng Wang wrote:
> 
> 
> On 2025/10/13 23:58, David Hildenbrand wrote:
>> On 13.10.25 14:15, Kefeng Wang wrote:
>>> The prot_numa_skip() checks should be suitable for pmd folio too,
>>> which helps to avoid unnecessary pmd change and folio migration
>>> attempts.
>>>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> ---
>>>    mm/huge_memory.c | 21 +++++++--------------
>>>    mm/internal.h    |  2 ++
>>>    mm/mprotect.c    |  2 +-
>>>    3 files changed, 10 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 1b81680b4225..feca5a19104a 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2395,8 +2395,7 @@ int change_huge_pmd(struct mmu_gather *tlb,
>>> struct vm_area_struct *vma,
>>>    #endif
>>>        if (prot_numa) {
>>> -        struct folio *folio;
>>> -        bool toptier;
>>> +        int target_node = NUMA_NO_NODE;
>>>            /*
>>>             * Avoid trapping faults against the zero page. The read-only
>>>             * data is likely to be read-cached on the local CPU and
>>> @@ -2408,19 +2407,13 @@ int change_huge_pmd(struct mmu_gather *tlb,
>>> struct vm_area_struct *vma,
>>>            if (pmd_protnone(*pmd))
>>>                goto unlock;
>>> -        folio = pmd_folio(*pmd);
>>> -        toptier = node_is_toptier(folio_nid(folio));
>>> -        /*
>>> -         * Skip scanning top tier node if normal numa
>>> -         * balancing is disabled
>>> -         */
>>> -        if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) &&
>>> -            toptier)
>>> -            goto unlock;
>>> +        /* Get target node for single threaded private VMAs */
>>> +        if (!(vma->vm_flags & VM_SHARED) &&
>>> +            atomic_read(&vma->vm_mm->mm_users) == 1)
>>> +            target_node = numa_node_id();
>>
>> Wondering if we should move that hunk into prot_numa_skip() as well.
>>
>> I'd assume numa_node_id() is not particularly expensive?
> 
> There is a atomic read, it may be expensive for 512 times atomic read
> for change_pte_range().

Note that atomic reads are usually just ordinary reads.

IIRC, the relevant code is

	cpu_to_node(raw_smp_processor_id())

And raw_smp_processor_id() is mostly either "current_thread_info()->cpu" 
or "this_cpu_read(cpu_number)".

But yeah, we can leave it as is for now and cache it in the caller.

-- 
Cheers

David / dhildenb