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How can I directly access reserved memory using the kernel module?

I am trying to limit the OS (Ubuntu Server 15.04) to a specific memory usage and reserve the rest, but write a kernel module to read / write to the reserved memory. I figured out how to limit the use / reserve memory using the kernel parameters "mem = 4G memmap = 4G @ 0 memmap = 4G $ 4G" (4 GB for the OS and 4 GB are reserved, divided into 4 GB points), but I don’t know how DMA for reserved memory works with kernel modules. I was thinking of just creating a proc file, but I'm not sure if you can create it outside of a dedicated OS.

Any suggestions? Thank!

Edit: This is for research, so there is no need to be "enjoyable."

Update: Perhaps I do not need to write a kernel module. I just found this and I'm going to take a picture: http://elinux.org/Memory_Management#Reserving_.28and_accessing.29_the_top_of_memory_on_startup

Update: I tried the link above, but I segfault whenever I try to write. Here is my code:

    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/mann.h>

    #define RESERVED_MEMORY_SIZE 0x100000000

    int main() {
            int fd;
            char *reserved_memory;

            fd = open("/dev/mem", O_RDWR | O_SYNC);
            reserved_memory = (char *) mmap(0, RESERVED_MEMORY_SIZE, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, 4096);
            reserved_memory[0] = 'a';
            return 0;
    }

dmesg shows:

    a.out[1167]: segfault at ffffffffffffffff ip 00000000004005d7 sp 00007ffeffccbd80 error 7 in a.out[400000+1000]

For hits I tried reserved_memory [1]:

    a.out[1180]: segfault at 0 ip 00000000004005db sp 00007ffc388d77b0 error 6 in a.out[400000+1000]

I will review the format of these messages to understand what he is telling me.

Update:

I found this question someone with the same problem as me, but the only solution is to restore the kernel. I will try to avoid this, so perhaps my best option is a custom kernel module. access to mmaped / dev / mem?

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, , . , , mmap, , ​​ //dev/mem, . , .

"Hello World!":

    #include <errno.h>
    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/mman.h>

    #define RESERVED_MEMORY_OFFSET  0x100000000     /* Offset is 4GB */

    int main() {
            int fd;
            char *reserved_memory;
            char *buffer = "Hello World!";

            fd = open("/dev/mem", O_RDWR | O_SYNC):
            /* Returns a pointer to the 4GB point in /dev/mem - the start of my reserved memory. Only mapping 4096 bytes. */
            reserved_memory = (char *) mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, RESERVED_MEMORY_OFFSET);
            if (reserved_memory == MAP_FAILED) {
                    printf("Failed to creating mapping.\n");
                    printf("ERRNO: %s\n", strerror(errno));
                    return -1;
            }
            sprintf(reserved_memory, "%s", buffer);
            return 0;
    }

:

    #include <errno.h>
    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/mman.h>

    #define RESERVED_MEMORY_OFFSET  0x100000000     /* Offset is 4GB */

    int main() {
            int fd;
            char *reserved_memory;
            char buffer[13];

            fd = open("/dev/mem", O_RDWR | O_SYNC):
            /* Returns a pointer to the 4GB point in /dev/mem - the start of my reserved memory. Only mapping 4096 bytes. */
            reserved_memory = (char *) mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, RESERVED_MEMORY_OFFSET);
            if (reserved_memory == MAP_FAILED) {
                    printf("Failed to creating mapping.\n");
                    printf("ERRNO: %s\n", strerror(errno));
                    return -1;
            }
            snprintf(buffer, 13, "%s", reserved_memory);
            printf("%s\n", buffer);
            return 0;
    }

@knm241!

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