Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

[Dev Jain <dev.jain@arm.com>]

On 06/06/25 10:19 pm, Dev Jain wrote:
>
> On 05/06/25 1:30 pm, Baolin Wang wrote:
>> The MADV_COLLAPSE will ignore the system-wide Anon THP sysfs 
>> settings, which
>> means that even though we have disabled the Anon THP configuration, 
>> MADV_COLLAPSE
>> will still attempt to collapse into a Anon THP. This violates the 
>> rule we have
>> agreed upon: never means never.
>>
>> Another rule for madvise, referring to David's suggestion: “allowing 
>> for collapsing
>> in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
>>
>> To address this issue, should check whether the Anon THP 
>> configuration is disabled
>> in thp_vma_allowable_orders(), even when the TVA_ENFORCE_SYSFS flag 
>> is set.
>
> Did you mean to say "even when the TVA_ENFORCE_SYSFS flag is *not* 
> set"? Because if
> is set, then we have to check the anon THP sysfs config.

Otherwise the patch itself looks good to me, so:

Reviewed-by: Dev Jain <dev.jain@arm.com>


>
>>
>> In summary, the current strategy is:
>>
>> 1. If always & orders == 0, and madvise & orders == 0, and 
>> hugepage_global_enabled() == false
>> (global THP settings are not enabled), it means mTHP of that orders 
>> are prohibited
>> from being used, then madvise_collapse() is forbidden for that orders.
>>
>> 2. If always & orders == 0, and madvise & orders == 0, and 
>> hugepage_global_enabled() == true
>> (global THP settings are enabled), and inherit & orders == 0, it 
>> means mTHP of that
>> orders are still prohibited from being used, thus madvise_collapse() 
>> is not allowed
>> for that orders.
>>
>> Reviewed-by: Zi Yan <ziy@nvidia.com>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   include/linux/huge_mm.h | 23 +++++++++++++++++++----
>>   1 file changed, 19 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 2f190c90192d..199ddc9f04a1 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -287,20 +287,35 @@ unsigned long thp_vma_allowable_orders(struct 
>> vm_area_struct *vma,
>>                          unsigned long orders)
>>   {
>>       /* Optimization to check if required orders are enabled early. */
>> -    if ((tva_flags & TVA_ENFORCE_SYSFS) && vma_is_anonymous(vma)) {
>> -        unsigned long mask = READ_ONCE(huge_anon_orders_always);
>> +    if (vma_is_anonymous(vma)) {
>> +        unsigned long always = READ_ONCE(huge_anon_orders_always);
>> +        unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
>> +        unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);
>> +        unsigned long mask = always | madvise;
>> +
>> +        /*
>> +         * If the system-wide THP/mTHP sysfs settings are disabled,
>> +         * then we should never allow hugepages.
>> +         */
>> +        if (!(mask & orders) && !(hugepage_global_enabled() && 
>> (inherit & orders)))
>> +            return 0;
>> +
>> +        if (!(tva_flags & TVA_ENFORCE_SYSFS))
>> +            goto skip;
>>   +        mask = always;
>>           if (vm_flags & VM_HUGEPAGE)
>> -            mask |= READ_ONCE(huge_anon_orders_madvise);
>> +            mask |= madvise;
>>           if (hugepage_global_always() ||
>>               ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
>> -            mask |= READ_ONCE(huge_anon_orders_inherit);
>> +            mask |= inherit;
>>             orders &= mask;
>>           if (!orders)
>>               return 0;
>>       }
>>   +skip:
>>       return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, 
>> orders);
>>   }
>