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 15 Apr 2026, at 5:21, Baolin Wang wrote:

> On 4/15/26 4:00 PM, David Hildenbrand (Arm) wrote:
>> On 4/15/26 08:36, Baolin Wang wrote:
>>>
>>>
>>> On 4/15/26 2:25 AM, Zi Yan wrote:
>>>> On 14 Apr 2026, at 14:14, David Hildenbrand (Arm) wrote:
>>>>
>>>>>
>>>>> 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.
>>>>>
>>>>>
>>>>> I don't think there is other interaction with FS and the global toggle
>>>>> besides this and the one you are adjusting, right?
>>>>>
>>>
>>> I'm afraid not. Please also consider the per-size mTHP interfaces. It's
>>> possible that hugepage_global_enabled() returns false, but
>>> hugepages-2048kB/enabled is set to "always", which would still allow
>>> khugepaged to collapse folios.
>
> My comments are in reply to Zi’s comment:
>
> "I think hugepage_global_enabled() should be enough to decide whether khugepaged should run or not. "
>
> I’m concerned that only relying on hugepage_global_enabled() to decide whether khugepaged should run would cause a regression for anonymous and shmem memory collapse, as it ignores per-size mTHP configuration.
>
>> The question is really which semantics we want.
>>
>> Right now, there is no way to disable khugepaged for anon pages, to just
>> get them during page faults.
>
> Right.
>
>> And we are now talking about the same problem for FS: to only get them
>> during page faults (like we did so far without CONFIG_READ_ONLY_THP_FOR_FS).
>
> OK. I’m fine with using hugepage_global_enabled() to determine whether khugepaged scans file folios.
>
> My concern is that for anonymous memory and shmem, the per-size mTHP settings should be considered.

OK, I misunderstood the meaning of hugepage_global_enabled(), since per-size
mTHP settings could also enable khugepaged if PMD_SIZE is set.

I will take willy’s original suggestion and make khugepaged on if the global
setting is enabled. The below is the new version of this patch. I moved anon
pmd huge page code to a separate anon_hpage_pmd_enabled() like
shmem_hpage_pmd_enabled() and cleaned up the comment. Let me know your thoughts.

Thanks.

From 92b92f2b2ab41c70b41dd304ce648786ee6a1603 Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Wed, 15 Apr 2026 13:52:50 -0400
Subject: [PATCH] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check in
 hugepage_pmd_enabled()

Remove READ_ONLY_THP_FOR_FS and khugepaged for file-backed pmd-sized
hugepages are enabled by the global transparent hugepage control.
khugepaged can still be enabled by per-size control for anon and shmem when
the global control is off.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/khugepaged.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b8452dbdb043..586d27ce896e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -406,18 +406,8 @@ 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)
+static inline bool anon_hpage_pmd_enabled()
 {
-	/*
-	 * 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))
@@ -425,6 +415,20 @@ static bool hugepage_pmd_enabled(void)
 	if (test_bit(PMD_ORDER, &huge_anon_orders_inherit) &&
 	    hugepage_global_enabled())
 		return true;
+	return false;
+}
+
+static bool hugepage_pmd_enabled(void)
+{
+	/*
+	 * Anon, shmem and file-backed pmd-sized hugepages are all determined by
+	 * the global control. If the global control is off, anon and shmem
+	 * pmd-sized hugepages are also determined by its per-size control.
+	 */
+	if (hugepage_global_enabled())
+		return true;
+	if (anon_hpage_pmd_enabled())
+		return true;
 	if (IS_ENABLED(CONFIG_SHMEM) && shmem_hpage_pmd_enabled())
 		return true;
 	return false;
-- 
2.43.0



Best Regards,
Yan, Zi