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

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Message-ID: <B55442F0-2387-40B8-A980-F6F2B8FBA4D2@nvidia.com>
Date: Thu, 29 May 2025 22:22:56 -0400
From: Zi Yan <ziy@...dia.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled

On 29 May 2025, at 22:21, Baolin Wang wrote:

> On 2025/5/30 10:04, Zi Yan wrote:
>> On 29 May 2025, at 21:51, Baolin Wang wrote:
>>
>>> 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@...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)))
>>>>
>>>> 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().
>>
>> Got it. Setting always or madvise knob in per-size mTHP means user wants to
>> enable that size, so their orders are not limited by the global config.
>> Setting inherit means user wants to follow the global config.
>> Now it makes sense to me. I wonder if renaming inherit to inherit_global
>> and huge_anon_orders_inherit to huge_anon_orders_inherit_global
>> could make code more clear (We cannot change sysfs names, but changing
>> kernel variable names should be fine?).
>
> The 'huge_anon_orders_inherit' is also a per-size mTHP interface. See the doc:
> "
> Alternatively it is possible to specify that a given hugepage size
> will inherit the top-level "enabled" value::
>
>         echo inherit  >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
> "
>
> The 'inherit' already implies that it is meant to inherit the top-level 'enabled' value, so I personally still prefer the variable name 'inherit', just as we use it elsewhere.

OK. No problem.

>
>>> 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.
>>
>> Putting the summary in the comment might be very helpful. WDYT?
>
> Sure. will do.

Thanks.

>
>> Otherwise, the patch looks good to me. Thanks.
>>
>> Reviewed-by: Zi Yan <ziy@...dia.com>
>
> Thanks.


Best Regards,
Yan, Zi