linux-kernel - Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in khugepaged_scan_mm

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Message-ID: <a62b7461-3faf-494c-bd00-0206de184a5a@arm.com>
Date: Sun, 14 Sep 2025 21:46:26 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 david@...hat.com, lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 ioworker0@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in
 khugepaged_scan_mm_slot()


On 14/09/25 8:05 pm, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> Let's skip unsuitable VMAs early in the khugepaged scan; specifically,
> mlocked VMAs should not be touched.
>
> Note that the only other user of the VM_NO_KHUGEPAGED mask is
>   __thp_vma_allowable_orders(), which is also used by the MADV_COLLAPSE
> path. Since MADV_COLLAPSE has different rules (e.g., for mlocked VMAs), we
> cannot simply make the shared mask stricter as that would break it.
>
> So, we also introduce a new VM_NO_THP_COLLAPSE mask for that helper,
> leaving the stricter checks to be applied only within the khugepaged path
> itself.
>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   include/linux/mm.h |  6 +++++-
>   mm/huge_memory.c   |  2 +-
>   mm/khugepaged.c    | 14 +++++++++++++-
>   3 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index be3e6fb4d0db..cb54d94b2343 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -505,7 +505,11 @@ extern unsigned int kobjsize(const void *objp);
>   #define VM_REMAP_FLAGS (VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP)
>   
>   /* This mask prevents VMA from being scanned with khugepaged */
> -#define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB)
> +#define VM_NO_KHUGEPAGED \
> +	(VM_SPECIAL | VM_HUGETLB | VM_LOCKED_MASK | VM_NOHUGEPAGE)
> +
> +/* This mask prevents VMA from being collapsed by any THP path */
> +#define VM_NO_THP_COLLAPSE	(VM_SPECIAL | VM_HUGETLB)

VM_NO_KHUGEPAGED should then be defined as VM_NO_THP_COLLAPSE | VM_LOCKED_MASK | VM_NOHUGEPAGE.
But...

I believe that the eligibility checking for khugepaged collapse is the business of
thp_vma_allowable_order(). This functionality should be put there, we literally
have a TVA_KHUGEPAGED flag :)

>   
>   /* This mask defines which mm->def_flags a process can inherit its parent */
>   #define VM_INIT_DEF_MASK	VM_NOHUGEPAGE
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d6fc669e11c1..2e91526a037f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -134,7 +134,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>   	 * Must be checked after dax since some dax mappings may have
>   	 * VM_MIXEDMAP set.
>   	 */
> -	if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED))
> +	if (!in_pf && !smaps && (vm_flags & VM_NO_THP_COLLAPSE))
>   		return 0;
>   
>   	/*
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7c5ff1b23e93..e54f99bb0b57 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -345,6 +345,17 @@ struct attribute_group khugepaged_attr_group = {
>   };
>   #endif /* CONFIG_SYSFS */
>   
> +/**
> + * khugepaged_should_scan_vma - check if a VMA is a candidate for collapse
> + * @vm_flags: The flags of the VMA to check.
> + *
> + * Returns: true if the VMA should be scanned by khugepaged, false otherwise.
> + */
> +static inline bool khugepaged_should_scan_vma(vm_flags_t vm_flags)
> +{
> +	return !(vm_flags & VM_NO_KHUGEPAGED);
> +}
> +
>   int hugepage_madvise(struct vm_area_struct *vma,
>   		     vm_flags_t *vm_flags, int advice)
>   {
> @@ -2443,7 +2454,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>   			progress++;
>   			break;
>   		}
> -		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
> +		if (!khugepaged_should_scan_vma(vma->vm_flags) ||
> +		    !thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
>   skip:
>   			progress++;
>   			continue;