Re: [PATCH v3] mm: simplify find_vma_prev

[Michal Hocko]

On Fri 09-12-11 17:48:40, kosaki.motohiro@xxxxxxxxx wrote:
> From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> 
> commit 297c5eee37 (mm: make the vma list be doubly linked) added
> vm_prev member into vm_area_struct. Therefore we can simplify
> find_vma_prev() by using it. Also, this change help to improve
> page fault performance because it has strong locality of reference.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> ---
>  mm/mmap.c |   36 ++++++++----------------------------
>  1 files changed, 8 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index eae90af..b9c0241 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1603,39 +1603,19 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>  
>  EXPORT_SYMBOL(find_vma);
>  
> -/* Same as find_vma, but also return a pointer to the previous VMA in *pprev. */
> +/*
> + * Same as find_vma, but also return a pointer to the previous VMA in *pprev.
> + * Note: pprev is set to NULL when return value is NULL.
> + */
>  struct vm_area_struct *
>  find_vma_prev(struct mm_struct *mm, unsigned long addr,
>  			struct vm_area_struct **pprev)
>  {
> -	struct vm_area_struct *vma = NULL, *prev = NULL;
> -	struct rb_node *rb_node;
> -	if (!mm)
> -		goto out;
> -
> -	/* Guard against addr being lower than the first VMA */
> -	vma = mm->mmap;

Why have you removed this guard? Previously we had pprev==NULL and
returned mm->mmap.
This seems like a semantic change without any explanation. Could you
clarify?

> -
> -	/* Go through the RB tree quickly. */
> -	rb_node = mm->mm_rb.rb_node;
> -
> -	while (rb_node) {
> -		struct vm_area_struct *vma_tmp;
> -		vma_tmp = rb_entry(rb_node, struct vm_area_struct, vm_rb);
> -
> -		if (addr < vma_tmp->vm_end) {
> -			rb_node = rb_node->rb_left;
> -		} else {
> -			prev = vma_tmp;
> -			if (!prev->vm_next || (addr < prev->vm_next->vm_end))
> -				break;
> -			rb_node = rb_node->rb_right;
> -		}
> -	}
> +	struct vm_area_struct *vma;
>  
> -out:
> -	*pprev = prev;
> -	return prev ? prev->vm_next : vma;
> +	vma = find_vma(mm, addr);
> +	*pprev = vma ? vma->vm_prev : NULL;
> +	return vma;
>  }
>  
>  /*
> -- 
> 1.7.1
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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