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

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Message-ID: <a862f31a-bf85-46fe-aebb-ae8c78e5a983@linux.alibaba.com>
Date: Mon, 9 Jun 2025 14:36:01 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Nico Pache <npache@...hat.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, ryan.roberts@....com,
 dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled



On 2025/6/9 02:37, Nico Pache wrote:
> On Thu, Jun 5, 2025 at 2:01 AM Baolin Wang
> <baolin.wang@...ux.alibaba.com> 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.
>>
>> 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@...dia.com>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.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;
> Hi Baolin,
> 
> Thanks for making this change so we enforce the 'never' being never
> rule. As others have noted, this statement is hard to read-- is there
> any improvements we can make to the readability as Lorenzo suggested?

Yes, will do in next version.

> Either way, I've tested this and the functionality works as intended.
> 
> Tested-by: Nico Pache <npache@...hat.com>

Thanks for testing.