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Message-ID: <2ff65f37-efa9-4e96-9cdf-534d63ff154e@linux.alibaba.com>
Date: Thu, 12 Jun 2025 15:51:47 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: 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, 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 2025/6/11 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;
+ 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);
}
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