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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 4/16/26 2:01 AM, Zi Yan wrote:
> 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;

Thanks. Looks reasonable to me. Let’s also see what David thinks.