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max_low_pfn variable is incorrectly adjusted if the kernel is built with high memory support and the later is detected in a running system, so the memory which actually can be directly mapped is getting into the highmem zone. See the ZONE_NORMAL range on my MIPS32r5 system: > Zone ranges: > DMA [mem 0x0000000000000000-0x0000000000ffffff] > Normal [mem 0x0000000001000000-0x0000000007ffffff] > HighMem [mem 0x0000000008000000-0x000000020fffffff] while the zones are supposed to look as follows: > Zone ranges: > DMA [mem 0x0000000000000000-0x0000000000ffffff] > Normal [mem 0x0000000001000000-0x000000001fffffff] > HighMem [mem 0x0000000020000000-0x000000020fffffff] Even though the physical memory within the range [0x08000000;0x20000000] belongs to MMIO on our system, we don't really want it to be considered as high memory since on MIPS32 that range still can be directly mapped. Note there might be other problems caused by the max_low_pfn variable misconfiguration. For instance high_memory variable is initialize with virtual address corresponding to the max_low_pfn PFN, and by design it must define the upper bound on direct map memory, then end of the normal zone. That in its turn potentially may cause problems in accessing the memory by means of the /dev/mem and /dev/kmem devices. Let's fix the discovered misconfiguration then. It turns out the commit |
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