map uncached memory in userspace experiment.
How to use
User process requests uncached memory by calling mmap() on module's character device (see kern.log for major number).
type must be MAP_SHARED and size must be a multiple of page size.
See test/test.c for example doing read benchmarks on cached / uncached memory.
Demo
# make
make -C /lib/modules/4.4.0-45-generic/build/ M=/root/src/uncached_ram_lkm modules
make[1]: Entering directory `/usr/src/linux-headers-4.4.0-45-generic'
CC [M] /root/src/uncached_ram_lkm/uncached_ram.o
Building modules, stage 2.
MODPOST 1 modules
CC /root/src/uncached_ram_lkm/uncached_ram.mod.o
LD [M] /root/src/uncached_ram_lkm/uncached_ram.ko
make[1]: Leaving directory `/usr/src/linux-headers-4.4.0-45-generic'
# insmod uncached_ram.ko
# tail /var/log/kern.log
Jul 27 20:42:44 localhost kernel: [115315.205744] Uncached ram module loaded
Jul 27 20:42:44 localhost kernel: [115315.205764] Created char device, major: 249
# cd test
# mknod dev c 249 0
# make
gcc -Wall -ggdb3 -std=gnu99 test.c -o test
# ./test
usage: ./test cached uncached_mem_dev
./test uncached uncached_mem_dev
# ./test cached dev
cached mem test: 104856600 reads in 1.06s (sum: 0)
# ./test uncached dev
mmap()'ing dev
uncached mem test: 104856600 reads in 10.73s (sum: -138595026)