On Fri 24-08-12 17:08:36, Li Haifeng wrote:

2012/8/24 Michal Hocko [off-list ref]:

quoted

On Fri 24-08-12 10:08:20, Li Haifeng wrote:
[...]

quoted

Subject: [PATCH] Fix the page address of higher page's buddy calculation

Calculate the page address of higher page's buddy should be based
higher_page with the offset between index of higher page and
index of higher page's buddy.

Sorry for insisting but could you add an information about when this has
been introduced (I have mentioned the commit in the other email) and the
effect of the bug so that we can consider whether this is worth
backporting to stable trees.

quoted

Signed-off-by: Haifeng Li <redacted>
Signed-off-by: Gavin Shan <redacted>

Other than that
Reviewed-by: Michal Hocko <redacted>

quoted

---
 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 cdef1d4..642cd62 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -536,7 +536,7 @@ 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 = higher_page + (buddy_idx - combined_idx);
                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

I am sorry Michal. I misinterpreted what you mean.

And the post blow is OK?
------------------------------------------>
Subject: [PATCH] Fix the page address of higher page's buddy calculation

The heuristic method for buddy has been introduced since
43506fad(mm/page_alloc.c: simplify calculation of combined index
of adjacent buddy lists). 

Maybe I just misunderstood you here but the heuristic has been
introduced by 6dda9d55 (page allocator: reduce fragmentation in buddy
allocator by adding buddies that are merging to the tail of the free
lists) and the commit you are mentioning broke it.

But the page address of higher page's buddy was wrongly calculated,
which will lead page_is_buddy to fail for ever. IOW, the heuristic
method would be disabled with the wrong page address of higher page's
buddy.

Calculating the page address of higher page's buddy should be based
higher_page with the offset between index of higher page and
index of higher page's buddy.

I would also add CC: stable [2.6.38+] although it is not clear how much
the heuristic is helpful. Anyway it's a regression and should be fix
IMHO.

-- 
Michal Hocko
SUSE Labs

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