linux-kernel - Re: [PATCH v2] mm: simplify find_vma

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Date:	Fri, 09 Dec 2011 13:35:26 -0800
From:	Joe Perches <joe@...ches.com>
To:	kosaki.motohiro@...il.com
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Shaohua Li <shaohua.li@...el.com>
Subject: Re: [PATCH v2] mm: simplify find_vma_prev

On Fri, 2011-12-09 at 16:23 -0500, kosaki.motohiro@...il.com wrote:
> 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.

trivia:

> diff --git a/mm/mmap.c b/mm/mmap.c
[]
> @@ -1603,39 +1603,21 @@ 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)

> +find_vma_prev(struct mm_struct *mm, unsigned long addr, struct vm_area_struct **pprev)

eh.  This declaration change seems gratuitous and it exceeds 80 columns.

> +	*pprev = NULL;
> +	vma = find_vma(mm, addr);
> +	if (vma)
> +		*pprev = vma->vm_prev;

There's no need to possibly set *pprev twice.

Maybe
{
	struct vm_area_struct *vma = find_vma(mm, addr);

	*pprev = vma ? vma->vm_prev : NULL;
or
	if (vma)
		*pprev = vma->vm_prev;
	else
		*pprev = NULL;

	return vma;
}
 
> -out:
> -	*pprev = prev;
> -	return prev ? prev->vm_next : vma;
> +	return vma;
>  }
>  
>  /*



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