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Message-ID: <40d859c8-f083-4b25-8785-c1023da1c6d8@arm.com>
Date: Thu, 12 Jun 2025 14:16:47 +0530
From: Dev Jain <dev.jain@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
David Hildenbrand <david@...hat.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 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.
> + unsigned long mask = always | madvise;
> +
> + mask = always | madvise;
> + if (inherit_enabled)
> + mask |= inherit;
> +
> + /* All set to/inherit NEVER - never means never globally,
> abort. */
> + if (!(mask & orders))
> + return 0;
> +
> + /*
> + * Otherwise, we only enforce sysfs settings if asked. In
> addition,
> + * if the user sets a sysfs mode of madvise and if
> TVA_ENFORCE_SYSFS
> + * is not set, we don't bother checking whether the VMA has
> VM_HUGEPAGE
> + * set.
> + */
> + if (!(tva_flags & TVA_ENFORCE_SYSFS))
> + return orders;
> +
> + mask = always;
> + if (has_madvise)
> + mask |= madvise;
> + if (hugepage_global_always() || (has_madvise && inherit_enabled))
> + mask |= inherit;
> +
> + return orders & mask;
> +}
> +
> /**
> * thp_vma_allowable_orders - determine hugepage orders that are
> allowed for vma
> * @vma: the vm area to check
> @@ -287,19 +324,8 @@ 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 (vm_flags & VM_HUGEPAGE)
> - mask |= READ_ONCE(huge_anon_orders_madvise);
> - if (hugepage_global_always() ||
> - ((vm_flags & VM_HUGEPAGE) &&
> hugepage_global_enabled()))
> - mask |= READ_ONCE(huge_anon_orders_inherit);
> -
> - orders &= mask;
> - if (!orders)
> - return 0;
> - }
> + if (vma_is_anonymous(vma))
> + orders = __thp_mask_anon_orders(vm_flags, tva_flags,
> orders);
>
> return __thp_vma_allowable_orders(vma, vm_flags, tva_flags,
> orders);
> }
Overall this is very readable!
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