mmap CMA area on /dev/mem

Question

I need reserve 256-512 Mb of continuous physical memory and have access to this memory from the user space.
I decided to use CMA for memory reserving.
Here are the steps on my idea that must be performed:

1. Reservation required amount of memory by CMA during system booting.
2. Parsing of CMA patch output which looks like for example: "CMA: reserved 256 MiB at 27400000" and saving two parameters: size of CMA area = 256*1024*1024 bytes and phys address of CMA area = 0x27400000.
3. Mapping of CMA area at /dev/mem file with offset = 0x27400000 using mmap(). (Of course, CONFIG_STRICT_DEVMEM is disabled) It would let me to read data directly from phys memory from user space.

But the next code make segmentation fault(there size = 1Mb):

int file;
void* start;

file=open("/dev/mem", O_RDWR | O_SYNC);

if ( (start = mmap(0, 1024*1024, PROT_READ | PROT_WRITE, MAP_SHARED, file, 0x27400000)) == MAP_FAILED ){
    perror("mmap");
}

for (int offs = 0; offs<50; offs++){
    cout<<((char *)start)[offs];
}

Output of this code: "mmap: Invalid argument".

When I changed offset = 0x27400000 on 0, this code worked fine and program displayed trash. It also work for alot of offsets which I looked at /proc/iomem. According to information from /proc/iomem, phys addr of CMA area (0x27400000 on my system) always situated in System RAM.

Does anyone have any ideas, how to mmap CMA area on /dev/mem? What am I doing wrong? Thanks alot for any help!

Answer 1

Solution to this problem was suggested to me by Jeff Haran in kernelnewbies mailing list.
It was necessary to disable CONFIG_x86_PAT in .config and mmap() has started to work!

If CONFIG_X86_PAT is configured you will have problems mapping memory to user space. It basically implements the same restrictions as CONFIG_STRICT_DEVMEM.
Jeff Haran

Now I can mmap /dev/mem on any physical address I want.
But necessary to be careful:

Word of caution. CONFIG_X86_PAT was likely introduced for a reason. There may be some performance penalties for turning this off, though in my testing so far turning if off doesn’t seem to break anything.
Jeff Haran