Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled
From: Dev Jain
Date: Thu Jun 12 2025 - 04:48:01 EST
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@xxxxxxxxxx>
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
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!