Your kernel likely doesn't have CONFIG_HIGHMEM flagged
Refer to here
as an example (there are many more)... where it states:
It is advised to not enable CONFIG_HIGHMEM in the kernel to utilize
the extra 128 MB you get for your 1 GB RAM system. I/O Devices cannot
directly address high memory from PCI space, so bounce buffers have to
be used. Plus the virtual memory management and paging costs come with
If you do a `dmesg|grep HIGHMEM` you'll probably see something like the following:
Jun 7 23:36:30 hostname kernel: Warning only 896MB will be used.
Jun 7 23:36:30 hostname kernel: Use a HIGHMEM enabled kernel.