I think it is because if you try hard you can map a dma page via
/dev/mem and then the driver can release the dma page (at rmmod for
example) before you munmap /dev/mem, and then the page goes in the
freelist and it is allocated as pagecache.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to firstname.lastname@example.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/