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 16 Apr 2026, at 4:47, David Hildenbrand (Arm) wrote:

> On 4/15/26 20:01, Zi Yan wrote:
>> On 15 Apr 2026, at 5:21, Baolin Wang wrote:
>>
>>> On 4/15/26 4:00 PM, David Hildenbrand (Arm) wrote:
>>>
>>> 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;
>> +}
>> +
>
> Works for me.
>
>> +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())
>
> BTW, can we please provide a stub for shmem_hpage_pmd_enabled() in
> shmem_fs.h such that we can remove the IS_ENABLED here?
>

Sure.

Best Regards,
Yan, Zi