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

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Sep 03, 2025 at 10:13:35AM +0200, David Hildenbrand wrote:
>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.
>

It seems reasonable, so

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

>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

-- 
Wei Yang
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