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 - 09:15:14 EST
On 12.06.25 15:07, Lorenzo Stoakes wrote:
On Thu, Jun 12, 2025 at 10:51:17AM +0200, David Hildenbrand wrote:
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 did wonder if we should call 'orders' something like 'requested_orders'
or something.
This check is always against the input orders which we might conceivably
want.
For instance in madvise_collapse():
if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER))
return -EINVAL;
I don't think, if it's only possible for PMD order collapse, and that is
disabled, but mTHP 64 KB let's say is enabled, it'd be fine for
MADV_COLLAPSE to succeed at the PMD order.
I would rewrite the function to only ever substract from "orders".
Hm.
...
/* Disallow orders that are set to NEVER directly ... */
order &= (always | madvise | inherit);
^s
I think you mean (always | madvise) here.
/* ... or through inheritance. */
if (inherit_enabled)
orders &= ~inherit;
order & (inherit & ~inherit) is going to always be zero :)
So this should be
orders &= inherit.
The problem is, when you come to the next stage where you are done checking
the 'are we in a NEVER situation regardless of TVA_ENFORCE_SYSFS' you've
now corrupted orders.
Because you've included inherit even if !(tva_flags & TVA_ENFORCE_SYSFS).
And there's no way to recover that information.
/*
* 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;
I don't think this is much delta to what we have now.
I do wonder if we should return mask & orders here, actually, to account
for the fact that, in theory, orders could set > PMD for
!TVA_ENFORCE_SYSFS) (not currently the case).
+
+ /*
+ * 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);
Obviously orders is corrupted at this point so this won't work, but I'm not
sure this is right?
If no madvise, only then obey always/inherit? Hm?
/* ... or through inheritance. */
if (!hugepage_global_always())
orders &= ~inherit;
I'm not sure about this ~inherit again, that means we ignore inherit no?
}
And we need a branch for madvise too no?
I think all of this is a _clear_ example of what a mess all this is.
I think we need a deeper refactor, but I think my suggested changes make at
least what we have here less horrid to get through.
I think maybe a better refactoring that's in the spirit of this is:
if (has_madvise) {
mask |= madvise;
if (inherit_enabled)
mask |= inherit;
} else if (hugepage_global_always()) {
mask |= inherit;
}
What do you think?
The masks are just disgusting :(
While you were writing that reply, I just sent out something else. Not
sure if that makes sense.
I'm having a hard time figuring out what we even want here in the
existing code.
--
Cheers,
David / dhildenb