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.

This was seen on 2.6.34.

Signed-off-by: Kevin Cernekee <[e-mail]>
To: Ralf Baechle <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/1992/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/mm/init.c

···
 #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