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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/5/29 23:10, Zi Yan wrote:
> On 29 May 2025, at 4:23, 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.
>>
>> 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.
>>
>> 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)))
> 
> Can you explain the logic here? Is it equivalent to:
> 1. if THP is set to always, always_mask & orders == 0, or
> 2. if THP if set to madvise, madvise_mask & order == 0, or
> 3. if THP is set to inherit, inherit_mask & order == 0?
> 
> I cannot figure out why (always | madvise) & orders does not check
> THP enablement case, but inherit & orders checks hugepage_global_enabled().

Sorry for not being clear. Let me try again:

Now we can control per-sized mTHP through ‘huge_anon_orders_always’, so 
always does not need to rely on the check of hugepage_global_enabled().

For madvise, referring to David's suggestion: “allowing for collapsing 
in a VM without VM_HUGEPAGE in the "madvise" mode would be fine", so we 
can just check 'huge_anon_orders_madvise' without relying on 
hugepage_global_enabled().

In the case where hugepage_global_enabled() is enabled, we need to check 
whether the 'inherit' has enabled the corresponding orders.

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 the orders are prohibited from being used, 
then madvise_collapse() is forbidden.

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 the orders are still prohibited 
from being used, and thus madvise_collapse() is not allowed.