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

[Zi Yan]

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.