Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

From: David Hildenbrand
Date: Thu Jun 12 2025 - 04:51:35 EST

On 12.06.25 09:51, 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;
+ 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;

Still confusing. I am not sure if we would properly catch when someone specifies e.g., 2M and 1M, while we only have 2M disabled.


I would rewrite the function to only ever substract from "orders".

...

/* Disallow orders that are set to NEVER directly ... */
order &= (always | madvise | inherit);

/* ... or through inheritance. */
if (inherit_enabled)
orders &= ~inherit;

/*
* 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 (!orders || !(tva_flags & TVA_ENFORCE_SYSFS))
return orders;

+
+ /*
+ * 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;

Similarly, this can maybe become (not 100% sure if I got it right, the condition above is confusing)

if (!has_madvise) {
/*
* Without VM_HUGEPAGE, only allow orders that are set to
* ALWAYS directly ...
*/
order &= (always | inherit);
/* ... or through inheritance. */
if (!hugepage_global_always())
orders &= ~inherit;
}

--
Cheers,

David / dhildenb