linux-kernel - [PATCH v2 2/6] MIPS: pfn_valid() is broken on low memory HIGHMEM systems

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Message-Id: <3b5632228986cbd69eae8787dd0aad5b@localhost>
Date:	Thu, 13 Jan 2011 17:52:13 -0800
From:	Kevin Cernekee <cernekee@...il.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
Cc:	<ddaney@...iumnetworks.com>, <linux-mips@...ux-mips.org>,
	<linux-kernel@...r.kernel.org>
Subject: [PATCH v2 2/6] MIPS: pfn_valid() is broken on low memory HIGHMEM systems

pfn_valid() compares the PFN to max_mapnr:

        __pfn >= min_low_pfn && __pfn < max_mapnr;

On HIGHMEM kernels, highend_pfn is used to set the value of max_mapnr.
Unfortunately, highend_pfn is left at zero if the system does not
actually have enough RAM to reach into the HIGHMEM range.  This causes
pfn_valid() to always return false, and when debug checks are enabled
the kernel will fail catastrophically:

Memory: 22432k/32768k available (2249k kernel code, 10336k reserved, 653k data, 1352k init, 0k highmem)
NR_IRQS:128
kfree_debugcheck: out of range ptr 81c02900h.
Kernel bug detected[#1]:
Cpu 0
$ 0   : 00000000 10008400 00000034 00000000
$ 4   : 8003e160 802a0000 8003e160 00000000
$ 8   : 00000000 0000003e 00000747 00000747
...

On such a configuration, max_low_pfn should be used to set max_mapnr.

Signed-off-by: Kevin Cernekee <cernekee@...il.com>
---
 arch/mips/mm/init.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/mips/mm/init.c b/arch/mips/mm/init.c
index 2efcbd2..45447cd 100644
--- a/arch/mips/mm/init.c
+++ b/arch/mips/mm/init.c
@@ -370,7 +370,7 @@ void __init mem_init(void)
 #ifdef CONFIG_DISCONTIGMEM
 #error "CONFIG_HIGHMEM and CONFIG_DISCONTIGMEM dont work together yet"
 #endif
-	max_mapnr = highend_pfn;
+	max_mapnr = highend_pfn ? highend_pfn : max_low_pfn;
 #else
 	max_mapnr = max_low_pfn;
 #endif
-- 
1.7.0.4

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