linux-kernel - Re: Fixup the page of buddy_higher address's calculation

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Date:	Thu, 23 Aug 2012 18:21:06 +0800
From:	Li Haifeng <omycle@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <jweiner@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: Fixup the page of buddy_higher address's calculation

I am sorry for my mistake.

higher_buddy is corresponding with buddy_index, and higher page is
corresponding with combined_idx. That is right.

But, How we get the page address from index offset? The key answer is
what is the base value.
So calculating the address based page should be (page + (buddy_idx - page_idx)).

Maybe, a diagram is easier to understand.

 |-------------------------|-------------|
page               combined   buddy

buddy's page address= page‘s page address + (buddy - page)*sizeof(struct page)

Clear?

2012/8/23 Michal Hocko <mhocko@...e.cz>:
> On Thu 23-08-12 16:40:13, Li Haifeng wrote:
>> From d7cd78f9d71a5c9ddeed02724558096f0bb4508a Mon Sep 17 00:00:00 2001
>> From: Haifeng Li <omycle@...il.com>
>> Date: Thu, 23 Aug 2012 16:27:19 +0800
>> Subject: [PATCH] Fixup the page of buddy_higher address's calculation
>
> Some general questions:
> Any word about the change? Is it really that obvious? Why do you think the
> current state is incorrect? How did you find out?
>
> And more specific below:
>
>> Signed-off-by: Haifeng Li <omycle@...il.com>
>> ---
>>  mm/page_alloc.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index ddbc17d..5588f68 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -579,7 +579,7 @@ static inline void __free_one_page(struct page *page,
>>                 combined_idx = buddy_idx & page_idx;
>>                 higher_page = page + (combined_idx - page_idx);
>>                 buddy_idx = __find_buddy_index(combined_idx, order + 1);
>> -               higher_buddy = page + (buddy_idx - combined_idx);
>> +               higher_buddy = page + (buddy_idx - page_idx);
>
> We are finding buddy index for combined_idx so why should we use
> page_idx here?
>
>>                 if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
>>                         list_add_tail(&page->lru,
>>                                 &zone->free_area[order].free_list[migratetype]);
>> --
>> 1.7.5.4
>
> --
> Michal Hocko
> SUSE Labs
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