Re: [PATCH 7.2 v2 05/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check in hugepage_pmd_enabled()

[Zi Yan <ziy@nvidia.com>]

On 14 Apr 2026, at 14:14, David Hildenbrand (Arm) wrote:

> On 4/14/26 18:30, Zi Yan wrote:
>> On 14 Apr 2026, at 7:02, David Hildenbrand (Arm) wrote:
>>
>>> On 4/13/26 22:42, Zi Yan wrote:
>>>>
>>>>
>>>
>>> I assume such a change should come before patch #4, as it seems to affect
>>> the functionality that depended on CONFIG_READ_ONLY_THP_FOR_FS.
>>
>> If the goal is to have a knob of khugepaged for all files, yes I will move
>> the change before Patch 4.
>>
>>>
>>>> I thought about this, but it means khugepaged is turned on regardless of
>>>> anon and shmem configs. I tend to think the original code was a bug,
>>>> since enabling CONFIG_READ_ONLY_THP_FOR_FS would enable khugepaged all
>>>> the time.
>>>
>>> There might be some FS mapping to collapse? So that makes sense to
>>> some degree.
>>>
>>> I really don't like the side-effects of "/sys/kernel/mm/transparent_hugepage/enabled".
>>> Like, enabling khugepaged+PMD for files.
>>>
>>
>> I am not a fan either, but I was not sure about another sysfs knob.
>>
>
> Yeah, it would be better if we could avoid it. But the dependency on the
> global toggle as it is today is a bit weird.
>
>>>>
>>>>
>>>> Alternatives could be:
>>>> 1. to add a file-backed khhugepaged config, but another sysfs?
>>>
>>> Maybe that would be the time to decouple file THP logic from
>>> hugepage_global_enabled()/hugepage_global_always().
>>>
>>> In particular, as pagecache folio allocation doesn't really care about __thp_vma_allowable_orders() IIRC.
>>>
>>> I'm thinking about something like the following:
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index b2a6060b3c20..fb3a4fd84fe0 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -184,15 +184,6 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>>                                                    forced_collapse);
>>>
>>>         if (!vma_is_anonymous(vma)) {
>>> -               /*
>>> -                * Enforce THP collapse requirements as necessary. Anonymous vmas
>>> -                * were already handled in thp_vma_allowable_orders().
>>> -                */
>>> -               if (!forced_collapse &&
>>> -                   (!hugepage_global_enabled() || (!(vm_flags & VM_HUGEPAGE) &&
>>> -                                                   !hugepage_global_always())))
>>> -                       return 0;
>>> -
>>>                 /*
>>>                  * Trust that ->huge_fault() handlers know what they are doing
>>>                  * in fault path.
>>
>> Looks reasonable.
>
> I don't think there is other interaction with FS and the global toggle
> besides this and the one you are adjusting, right?
>
>>
>>>
>>> Then, we might indeed just want a khugepaged toggle whether to enable it at
>>> all in files. (or just a toggle to disable khugeapged entirely?)
>>>
>>
>> I think hugepage_global_enabled() should be enough to decide whether khugepaged
>> should run or not.
>
> That would also be an option and would likely avoid other toggles.
>
> So __thp_vma_allowable_orders() would allows THPs in any case for FS,
> but hugepage_global_enabled() would control whether khugepaged runs (for
> fs).
>
> It gives less flexibility, but likely that's ok.
>
>>
>> Currently, we have thp_vma_allowable_orders() to filter each VMAs and I do not
>> see a reason to use hugepage_pmd_enabled() to guard khugepaged daemon. I am
>> going to just remove hugepage_pmd_enabled() and replace it with
>> hugepage_global_enabled(). Let me know your thoughts.
>
> Can you send a quick draft of what you have in mind?

From ee9e1c18b41111db7248db7fb64693b91e32255d Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Tue, 14 Apr 2026 14:17:31 -0400
Subject: [PATCH] mm/khugepaged: replace hugepage_pmd_enabled with
 hugepage_global_enabled

thp_vma_allowable_orders() is used to guard khugepaged scanning logic in
collapse_scan_mm_slot() based on enabled THP/mTHP orders by only allowing
PMD_ORDER. hugepage_pmd_enabled() is a duplication of it for khugepaged
start/stop control. Simplify the control by checking
hugepage_global_enabled() instead and let thp_vma_allowable_orders() filter
khugepaged scanning.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/khugepaged.c | 36 ++++++------------------------------
 1 file changed, 6 insertions(+), 30 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b8452dbdb043..459c486a5a75 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -406,30 +406,6 @@ static inline int collapse_test_exit_or_disable(struct mm_struct *mm)
 		mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
 }

-static bool hugepage_pmd_enabled(void)
-{
-	/*
-	 * We cover the anon, shmem and the file-backed case here; file-backed
-	 * hugepages, when configured in, are determined by the global control.
-	 * Anon pmd-sized hugepages are determined by the pmd-size control.
-	 * Shmem pmd-sized hugepages are also determined by its pmd-size control,
-	 * except when the global shmem_huge is set to SHMEM_HUGE_DENY.
-	 */
-	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
-	    hugepage_global_enabled())
-		return true;
-	if (test_bit(PMD_ORDER, &huge_anon_orders_always))
-		return true;
-	if (test_bit(PMD_ORDER, &huge_anon_orders_madvise))
-		return true;
-	if (test_bit(PMD_ORDER, &huge_anon_orders_inherit) &&
-	    hugepage_global_enabled())
-		return true;
-	if (IS_ENABLED(CONFIG_SHMEM) && shmem_hpage_pmd_enabled())
-		return true;
-	return false;
-}
-
 void __khugepaged_enter(struct mm_struct *mm)
 {
 	struct mm_slot *slot;
@@ -463,7 +439,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
 			  vm_flags_t vm_flags)
 {
 	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
-	    hugepage_pmd_enabled()) {
+	    hugepage_global_enabled()) {
 		if (thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
 			__khugepaged_enter(vma->vm_mm);
 	}
@@ -2599,7 +2575,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,

 static int khugepaged_has_work(void)
 {
-	return !list_empty(&khugepaged_scan.mm_head) && hugepage_pmd_enabled();
+	return !list_empty(&khugepaged_scan.mm_head) && hugepage_global_enabled();
 }

 static int khugepaged_wait_event(void)
@@ -2672,7 +2648,7 @@ static void khugepaged_wait_work(void)
 		return;
 	}

-	if (hugepage_pmd_enabled())
+	if (hugepage_global_enabled())
 		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
 }

@@ -2703,7 +2679,7 @@ void set_recommended_min_free_kbytes(void)
 	int nr_zones = 0;
 	unsigned long recommended_min;

-	if (!hugepage_pmd_enabled()) {
+	if (!hugepage_global_enabled()) {
 		calculate_min_free_kbytes();
 		goto update_wmarks;
 	}
@@ -2753,7 +2729,7 @@ int start_stop_khugepaged(void)
 	int err = 0;

 	mutex_lock(&khugepaged_mutex);
-	if (hugepage_pmd_enabled()) {
+	if (hugepage_global_enabled()) {
 		if (!khugepaged_thread)
 			khugepaged_thread = kthread_run(khugepaged, NULL,
 							"khugepaged");
@@ -2779,7 +2755,7 @@ int start_stop_khugepaged(void)
 void khugepaged_min_free_kbytes_update(void)
 {
 	mutex_lock(&khugepaged_mutex);
-	if (hugepage_pmd_enabled() && khugepaged_thread)
+	if (hugepage_global_enabled() && khugepaged_thread)
 		set_recommended_min_free_kbytes();
 	mutex_unlock(&khugepaged_mutex);
 }
-- 
2.43.0



>
>>
>> BTW, this conflicts with Patch 12 from Nico’s khugepaged for mTHP patchset.
>
> Right. I guess it can be handled.
>
> -- 
> Cheers,
>
> David


Best Regards,
Yan, Zi