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

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Message-ID: <99f8cb5e-828a-444c-b207-2a12e13a45f5@redhat.com>
Date: Wed, 3 Sep 2025 11:22:09 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, Wei Yang <richard.weiyang@...il.com>
Cc: akpm@...ux-foundation.org, kas@...nel.org, willy@...radead.org,
 hughd@...gle.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, baohua@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs

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@....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