linux-kernel - Re: [PATCH v10 09/13] khugepaged: enable collapsing mTHPs even when PMD THPs are disabled

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Message-ID: <af8a7c77-fc79-4c9e-93e7-f69300580a8d@lucifer.local>
Date: Thu, 21 Aug 2025 14:35:28 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Nico Pache <npache@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
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Subject: Re: [PATCH v10 09/13] khugepaged: enable collapsing mTHPs even when
 PMD THPs are disabled

On Tue, Aug 19, 2025 at 07:42:01AM -0600, Nico Pache wrote:
> From: Baolin Wang <baolin.wang@...ux.alibaba.com>
>
> We have now allowed mTHP collapse, but thp_vma_allowable_order() still only
> checks if the PMD-sized mTHP is allowed to collapse. This prevents scanning
> and collapsing of 64K mTHP when only 64K mTHP is enabled. Thus, we should
> modify the checks to allow all large orders of anonymous mTHP.
>
> Acked-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  mm/khugepaged.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7d9b5100bea1..2cadd07341de 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -491,7 +491,11 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
>  {
>  	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
>  	    hugepage_pmd_enabled()) {
> -		if (thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
> +		unsigned long orders = vma_is_anonymous(vma) ?
> +					THP_ORDERS_ALL_ANON : BIT(PMD_ORDER);

We need some explanation here please, a comment explaining what's going on here
would go a long way.

> +
> +		if (thp_vma_allowable_orders(vma, vm_flags, TVA_KHUGEPAGED,
> +					    orders))
>  			__khugepaged_enter(vma->vm_mm);
>  	}
>  }
> @@ -2671,6 +2675,8 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
>
>  	vma_iter_init(&vmi, mm, khugepaged_scan.address);
>  	for_each_vma(vmi, vma) {
> +		unsigned long orders = vma_is_anonymous(vma) ?
> +					THP_ORDERS_ALL_ANON : BIT(PMD_ORDER);


Can we have this as a separate helper function please? As you're now open-coding
this in two places.

In fact, you can put the comment I mention above there and have that document
what's happening here.

>  		unsigned long hstart, hend;
>
>  		cond_resched();
> @@ -2678,7 +2684,8 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
>  			progress++;
>  			break;
>  		}
> -		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
> +		if (!thp_vma_allowable_orders(vma, vma->vm_flags,
> +			TVA_KHUGEPAGED, orders)) {
>  skip:
>  			progress++;
>  			continue;
> --
> 2.50.1
>