linux-kernel - [RFC][PATCH] Fix area->nr_free-- went (-1) issue in buddy system

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Message-ID: <6d6a94c50612181901m1bfd9d1bsc2d9496ab24eb3f8@mail.gmail.com>
Date:	Tue, 19 Dec 2006 11:01:36 +0800
From:	Aubrey <aubreylee@...il.com>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
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@...il.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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