Re: [v4 PATCH 6/8] mm: khugepaged: make hugepage_vma_check() non-static

[Andrew Morton]

On Tue, 10 May 2022 13:32:20 -0700 Yang Shi <shy828301@xxxxxxxxx> wrote:

> The hugepage_vma_check() could be reused by khugepaged_enter() and
> khugepaged_enter_vma_merge(), but it is static in khugepaged.c.
> Make it non-static and declare it in khugepaged.h.
> 
> ..
>
> @@ -508,20 +508,13 @@ void __khugepaged_enter(struct mm_struct *mm)
>  void khugepaged_enter_vma_merge(struct vm_area_struct *vma,
>  			       unsigned long vm_flags)
>  {
> -	unsigned long hstart, hend;
> -
> -	/*
> -	 * khugepaged only supports read-only files for non-shmem files.
> -	 * khugepaged does not yet work on special mappings. And
> -	 * file-private shmem THP is not supported.
> -	 */
> -	if (!hugepage_vma_check(vma, vm_flags))
> -		return;
> -
> -	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> -	hend = vma->vm_end & HPAGE_PMD_MASK;
> -	if (hstart < hend)
> -		khugepaged_enter(vma, vm_flags);
> +	if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> +	    khugepaged_enabled() &&
> +	    (((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
> +	     (vma->vm_end & HPAGE_PMD_MASK))) {

Reviewing these bounds-checking tests is so hard :(  Can we simplify?

> +		if (hugepage_vma_check(vma, vm_flags))
> +			__khugepaged_enter(vma->vm_mm);
> +	}
>  }

void khugepaged_enter_vma(struct vm_area_struct *vma,
			  unsigned long vm_flags)
{
	if (test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags))
		return;
	if (!khugepaged_enabled())
		return;
	if (round_up(vma->vm_start, HPAGE_PMD_SIZE) >=
			(vma->vm_end & HPAGE_PMD_MASK))
		return;		/* vma is too small */
	if (!hugepage_vma_check(vma, vm_flags))
		return;
	__khugepaged_enter(vma->vm_mm);
}


Also, it might be slightly faster to have checked MMF_VM_HUGEPAGE
before khugepaged_enabled(), but it looks odd.  And it might be slower,
too - more pointer chasing.

I wish someone would document hugepage_vma_check().