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Subject[RFC][PATCH] Fix area->nr_free-- went (-1) issue in buddy system
Hi all,

When I setup two zones (NORMAL and DMA) in my system, I got the
following wired result from /proc/buddyinfo.
-----------------------------------------------------------------------------------------
root:~> cat /proc/buddyinfo
Node 0, zone DMA 2 1 2 1 1 0 0
1 1 2 2 0 0 0
Node 0, zone Normal 1 1 1 1 1 1 0
0 4294967295 0 4294967295 2 0 0
-----------------------------------------------------------------------------------------
As you see, two area->nr_free went -1.

After dig into the code, I found the problem is in the fun
__free_one_page() when the kernel boot up call free_all_bootmem(). If
two zones setup, it's possible NORMAL zone merged a block whose order
=8 at the first time(this time zone[NORMA]->free_area[8].nr_free = 0)
and found its buddy in the DMA zone. So the two blocks will be merged
and area->nr_free went to -1.

My proposed patch is as follows:


Signed-off-by: Aubrey Li <aubreylee@gmail.com>
---------------------------------------------------------------------------------------------------
--- page_alloc.c.orig 2006-12-19 10:45:25.000000000 +0800
+++ page_alloc.c 2006-12-19 10:44:48.000000000 +0800
@@ -407,7 +407,8 @@ static inline void __free_one_page(struc

list_del(&buddy->lru);
area = zone->free_area + order;
- area->nr_free--;
+ if (area->nr_free > 0)
+ area->nr_free--;
rmv_page_order(buddy);
combined_idx = __find_combined_index(page_idx, order);
page = page + (combined_idx - page_idx);
----------------------------------------------------------------------------------------------------
Any comments?
Thanks,
-Aubrey
-
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