Re: Fixup the page of buddy_higher address's calculation

[Gavin Shan <shangw@linux.vnet.ibm.com>]

On Thu, Aug 23, 2012 at 06:21:06PM +0800, Li Haifeng wrote:
>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= pagea??s page address + (buddy - page)*sizeof(struct page)
>
>Clear?
>

It sounds reasonable.

>2012/8/23 Michal Hocko <mhocko@suse.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@gmail.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@gmail.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);

Haifeng, Not sure it would be better? At least, the expression
would be more explicitly meaningful than yours.

		    higher_buddy = higher_page + (buddy_idx - combined_idx);

Thanks,
Gavin

>>
>> 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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