linux-kernel - Re: [PATCH v3] mm: simplify find_vma

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Date:	Mon, 12 Dec 2011 09:49:00 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
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 v3] mm: simplify find_vma_prev

(12/12/11 8:26 AM), Michal Hocko wrote:
> On Fri 09-12-11 17:48:40, kosaki.motohiro@...il.com wrote:
>> From: KOSAKI Motohiro<kosaki.motohiro@...fujitsu.com>
>>
>> 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@...fujitsu.com>
>> ---
>>   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?

IIUC, find_vma_prev() is module unexported and none of known caller use
pprev==NULL. Thus, I thought it can be also simplified. Am I missing 
something?

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