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

David Hildenbrand
2025-09-03T08:13:46Z

On 03.09.25 10:08, 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?

The max_ptes_shared check should handle that, so I don't immediately see
a problem with that.

When I thought about the "why is this writable check there" in the past,
I thought that maybe it was "smarter" to use THP where people are
actually using that memory for writing (writing heap etc).

But I can understand that some pure r/o users exists that can benefit as
well.

--
Cheers

David / dhildenb