Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs

David Hildenbrand
2025-09-03T09:22:20Z

On 03.09.25 11:18, Dev Jain wrote:
>
> On 03/09/25 2:45 pm, Dev Jain wrote:
>>
>> On 03/09/25 1:38 pm, Wei Yang wrote:
>>> On Wed, Sep 03, 2025 at 11:16:34AM +0530, Dev Jain wrote:
>>>> Currently khugepaged does not collapse a region which does not have a
>>>> single writable page. This is wasteful since non-writable VMAs
>>>> mapped by
>>>> the application won't benefit from THP collapse. Therefore, remove this
>>>> restriction and allow khugepaged to collapse a VMA with arbitrary
>>>> protections.
>>>>
>>>> Along with this, currently MADV_COLLAPSE does not perform a collapse
>>>> on a
>>>> non-writable VMA, and this restriction is nowhere to be found on the
>>>> manpage - the restriction itself sounds wrong to me since the user
>>>> knows
>>>> the protection of the memory it has mapped, so collapsing read-only
>>>> memory via madvise() should be a choice of the user which shouldn't
>>>> be overriden by the kernel.
>>>>
>>>> On an arm64 machine, an average of 5% improvement is seen on some
>>>> mmtests
>>>> benchmarks, particularly hackbench, with a maximum improvement of 12%.
>>>>
>>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>>> ---
>>> [...]
>>>> mm/khugepaged.c | 9 ++-------
>>>> 1 file changed, 2 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>> index 4ec324a4c1fe..a0f1df2a7ae6 100644
>>>> --- a/mm/khugepaged.c
>>>> +++ b/mm/khugepaged.c
>>>> @@ -676,9 +676,7 @@ static int __collapse_huge_page_isolate(struct
>>>> vm_area_struct *vma,
>>>>             writable = true;
>>>>     }
>>>>
>>>> -    if (unlikely(!writable)) {
>>>> -        result = SCAN_PAGE_RO;
>>>> -    } else if (unlikely(cc->is_khugepaged && !referenced)) {
>>> Would this cause more memory usage in system?
>>>
>>> For example, one application would fork itself many times. It
>>> executable area
>>> is read only, so all of them share one copy in memory.
>>>
>>> Now we may collapse the range and create one copy for each process.
>>>
>>> Ok, we have max_ptes_shared, while if some ptes are none, could it
>>> still do
>>> collapse?
>>>
>>> Maybe this is not realistic, just curious.
>>
>> Misunderstood your concern - you mean to say that a parent forks and
>> the children
>> VMAs are read-only pointing to the pages which were mapped by parent.
>> Hmm.
>
> I meant to say, writable VMAs with wrprotected ptes. Maybe after this
> patch, people
>
> can finally make some real use of the max_ptes_shared tunable :)

I hope not, because it should be burned with fire, lol :)

--
Cheers

David / dhildenb