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

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Message-ID: <20120823135839.GB19968@dhcp22.suse.cz>
Date:	Thu, 23 Aug 2012 15:58:39 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Gavin Shan <shangw@...ux.vnet.ibm.com>
Cc:	Li Haifeng <omycle@...il.com>,
	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

On Thu 23-08-12 20:30:34, Gavin Shan wrote:
> On Thu, Aug 23, 2012 at 06:21:06PM +0800, 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);
> 
> 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);

Yes, indeed. It would be also good to mention that this is a regression
since 43506fad (mm/page_alloc.c: simplify calculation of combined index
of adjacent buddy lists). IIUC this basically disables the heuristic
because page_is_buddy will fail for order+1, right?

Maybe 2.6.38+ stable candidate, then.

Could you repost with the full changelog, please?

Thanks
-- 
Michal Hocko
SUSE Labs
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