Re: [PATCH v8 mm-new 02/12] mm: thp: remove vm_flags parameter from khugepaged_enter_vma()

[Usama Arif <usamaarif642@gmail.com>]

On 26/09/2025 10:33, Yafang Shao wrote:
> The khugepaged_enter_vma() function requires handling in two specific
> scenarios:
> 1. New VMA creation
>   When a new VMA is created, if vma->vm_mm is not present in
>   khugepaged_mm_slot, it must be added. In this case,
>   khugepaged_enter_vma() is called after vma->vm_flags have been set,
>   allowing direct use of the VMA's flags.
> 2. VMA flag modification
>   When vma->vm_flags are modified (particularly when VM_HUGEPAGE is set),
>   the system must recheck whether to add vma->vm_mm to khugepaged_mm_slot.
>   Currently, khugepaged_enter_vma() is called before the flag update, so
>   the call must be relocated to occur after vma->vm_flags have been set.
> 
> Additionally, khugepaged_enter_vma() is invoked in other contexts, such as
> during VMA merging. However, these calls are unnecessary because the
> existing VMA already ensures that vma->vm_mm is registered in
> khugepaged_mm_slot. While removing these redundant calls represents a
> potential optimization, that change should be addressed separately.
> Because VMA merging only occurs when the vm_flags of both VMAs are
> identical (excluding special flags like VM_SOFTDIRTY), we can safely use
> target->vm_flags instead.
> 

The patch looks good to me, but if we are sure that khugepaged_enter_vma
is not needed in VMA merging case, we should remove it in this patch itself.
If the reason we are removing what flags are being considered when calling
khugepaged_enter_vma in VMA merging case is because the calls are unnecessary,
then we should just remove the calls and not modify them
(if its safe and functionally correct :))

> After this change, we can further remove vm_flags parameter from
> thp_vma_allowable_order(). That will be handled in a followup patch.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Yang Shi <shy828301@gmail.com>
> ---
>  include/linux/khugepaged.h |  6 ++----
>  mm/huge_memory.c           |  2 +-
>  mm/khugepaged.c            | 11 ++---------
>  mm/madvise.c               |  7 +++++++
>  mm/vma.c                   |  6 +++---
>  5 files changed, 15 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index f14680cd9854..b30814d3d665 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -13,8 +13,7 @@ extern void khugepaged_destroy(void);
>  extern int start_stop_khugepaged(void);
>  extern void __khugepaged_enter(struct mm_struct *mm);
>  extern void __khugepaged_exit(struct mm_struct *mm);
> -extern void khugepaged_enter_vma(struct vm_area_struct *vma,
> -				 vm_flags_t vm_flags);
> +extern void khugepaged_enter_vma(struct vm_area_struct *vma);
>  extern void khugepaged_enter_mm(struct mm_struct *mm);
>  extern void khugepaged_min_free_kbytes_update(void);
>  extern bool current_is_khugepaged(void);
> @@ -39,8 +38,7 @@ static inline void khugepaged_fork(struct mm_struct *mm, struct mm_struct *oldmm
>  static inline void khugepaged_exit(struct mm_struct *mm)
>  {
>  }
> -static inline void khugepaged_enter_vma(struct vm_area_struct *vma,
> -					vm_flags_t vm_flags)
> +static inline void khugepaged_enter_vma(struct vm_area_struct *vma)
>  {
>  }
>  static inline void khugepaged_enter_mm(struct mm_struct *mm)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1b81680b4225..ac6601f30e65 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1346,7 +1346,7 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>  	ret = vmf_anon_prepare(vmf);
>  	if (ret)
>  		return ret;
> -	khugepaged_enter_vma(vma, vma->vm_flags);
> +	khugepaged_enter_vma(vma);
>  
>  	if (!(vmf->flags & FAULT_FLAG_WRITE) &&
>  			!mm_forbids_zeropage(vma->vm_mm) &&
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index f47ac8c19447..04121ae7d18d 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -353,12 +353,6 @@ int hugepage_madvise(struct vm_area_struct *vma,
>  #endif
>  		*vm_flags &= ~VM_NOHUGEPAGE;
>  		*vm_flags |= VM_HUGEPAGE;
> -		/*
> -		 * If the vma become good for khugepaged to scan,
> -		 * register it here without waiting a page fault that
> -		 * may not happen any time soon.
> -		 */
> -		khugepaged_enter_vma(vma, *vm_flags);
>  		break;
>  	case MADV_NOHUGEPAGE:
>  		*vm_flags &= ~VM_HUGEPAGE;
> @@ -467,10 +461,9 @@ void khugepaged_enter_mm(struct mm_struct *mm)
>  	__khugepaged_enter(mm);
>  }
>  
> -void khugepaged_enter_vma(struct vm_area_struct *vma,
> -			  vm_flags_t vm_flags)
> +void khugepaged_enter_vma(struct vm_area_struct *vma)
>  {
> -	if (!thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
> +	if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
>  		return;
>  
>  	khugepaged_enter_mm(vma->vm_mm);
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 35ed4ab0d7c5..ab8b5d47badb 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1425,6 +1425,13 @@ static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)
>  	VM_WARN_ON_ONCE(madv_behavior->lock_mode != MADVISE_MMAP_WRITE_LOCK);
>  
>  	error = madvise_update_vma(new_flags, madv_behavior);
> +	/*
> +	 * If the vma become good for khugepaged to scan,
> +	 * register it here without waiting a page fault that
> +	 * may not happen any time soon.
> +	 */
> +	if (!error && new_flags & VM_HUGEPAGE)
> +		khugepaged_enter_mm(vma->vm_mm);
>  out:
>  	/*
>  	 * madvise() returns EAGAIN if kernel resources, such as
> diff --git a/mm/vma.c b/mm/vma.c
> index a1ec405bda25..6a548b0d64cd 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -973,7 +973,7 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
>  	if (err || commit_merge(vmg))
>  		goto abort;
>  
> -	khugepaged_enter_vma(vmg->target, vmg->vm_flags);
> +	khugepaged_enter_vma(vmg->target);
>  	vmg->state = VMA_MERGE_SUCCESS;
>  	return vmg->target;
>  
> @@ -1093,7 +1093,7 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
>  	 * following VMA if we have VMAs on both sides.
>  	 */
>  	if (vmg->target && !vma_expand(vmg)) {
> -		khugepaged_enter_vma(vmg->target, vmg->vm_flags);
> +		khugepaged_enter_vma(vmg->target);
>  		vmg->state = VMA_MERGE_SUCCESS;
>  		return vmg->target;
>  	}
> @@ -2520,7 +2520,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	 * call covers the non-merge case.
>  	 */
>  	if (!vma_is_anonymous(vma))
> -		khugepaged_enter_vma(vma, map->vm_flags);
> +		khugepaged_enter_vma(vma);
>  	*vmap = vma;
>  	return 0;
>