If an O_RDONLY open file is mapped PROT_READ, it will
not be coherent relative to writable mappings on other
nodes. This is related to base linux code that turns
off VM_SHARED if the file block is in FMODE_WRITE when
doing the mmap.
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This must be a 2.4 problem. If I read correctly 2.6.11-ssi base code doesn't turn off VM_SHARED when file is FMODE_WRITE.