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: <aa6db5d0-2cc0-4749-88ad-874fed0a168c@redhat.com>
Date: Thu, 12 Jun 2025 10:52:41 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 akpm@...ux-foundation.org, hughd@...gle.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....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 12.06.25 10:46, Dev Jain wrote:
> 
> On 12/06/25 1:21 pm, Baolin Wang wrote:
>>
>>
>> On 2025/6/11 20:34, David Hildenbrand wrote:
>>> On 05.06.25 10:00, 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.
>>>>
>>>> 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;
>>>
>>> I'm still trying to digest that. Isn't there a way for us to work
>>> with the orders,
>>> essentially masking off all orders that are forbidden globally.
>>> Similar to below, if !orders, then return 0?
>>> /* Orders disabled directly. */
>>> orders &= ~TODO;
>>> /* Orders disabled by inheriting from the global toggle. */
>>> if (!hugepage_global_enabled())
>>>       orders &= ~READ_ONCE(huge_anon_orders_inherit);
>>>
>>> TODO is probably a -1ULL and then clearing always/madvise/inherit.
>>> Could add a simple helper for that
>>>
>>> huge_anon_orders_never
>>
>> I followed Lorenzo's suggestion to simplify the logic. Does that look
>> more readable?
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 2f190c90192d..3087ac7631e0 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -265,6 +265,43 @@ unsigned long __thp_vma_allowable_orders(struct
>> vm_area_struct *vma,
>>                                           unsigned long tva_flags,
>>                                           unsigned long orders);
>>
>> +/* Strictly mask requested anonymous orders according to sysfs
>> settings. */
>> +static inline unsigned long __thp_mask_anon_orders(unsigned long
>> vm_flags,
>> +                               unsigned long tva_flags, unsigned long
>> orders)
>> +{
>> +       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);
>> +       bool inherit_enabled = hugepage_global_enabled();
>> +       bool has_madvise =  vm_flags & VM_HUGEPAGE;
> 
> Let us name this honour_madvise to indicate that this VMA must honour
> the madvise setting.

Not clear, because there is also "VM_NOHUGEPAGE" :/

See my reply where we maybe (if I got it right) only need a single check 
and can just avoid a confusing temporary variable completely.

-- 
Cheers,

David / dhildenb