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Message-ID: <0a52cb54-5633-4374-baa5-199194dfc2e1@arm.com>
Date: Wed, 3 Sep 2025 14:45:28 +0530
From: Dev Jain <dev.jain@....com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, 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 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.
>
>> + 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
>>
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