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

[Dev Jain <dev.jain@arm.com>]

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.

>
>> +	if (unlikely(cc->is_khugepaged && !referenced)) {
>> 		result = SCAN_LACK_REFERENCED_PAGE;
>> 	} else {
>> 		result = SCAN_SUCCEED;
>> @@ -1421,9 +1419,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>> 		     mmu_notifier_test_young(vma->vm_mm, _address)))
>> 			referenced++;
>> 	}
>> -	if (!writable) {
>> -		result = SCAN_PAGE_RO;
>> -	} else if (cc->is_khugepaged &&
>> +	if (cc->is_khugepaged &&
>> 		   (!referenced ||
>> 		    (unmapped && referenced < HPAGE_PMD_NR / 2))) {
>> 		result = SCAN_LACK_REFERENCED_PAGE;
>> @@ -2830,7 +2826,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>> 		case SCAN_PMD_NULL:
>> 		case SCAN_PTE_NON_PRESENT:
>> 		case SCAN_PTE_UFFD_WP:
>> -		case SCAN_PAGE_RO:
>> 		case SCAN_LACK_REFERENCED_PAGE:
>> 		case SCAN_PAGE_NULL:
>> 		case SCAN_PAGE_COUNT:
>> -- 
>> 2.30.2
>>