Dynamic memory allocation in Intel Assembly on Linux with sys_brk

So I am trying dynamic allocation of memory in C program with assembly (AT&T notation). The problem is that my allocation don't work. When I try too write a value into brk address I get segmentation fault (access violation). Also I compared brk address before and after allocation and they were the same so I guess it didn't worked. The information I want to add is that I want to allocate a bunch of memory pages. Code:

//...

    long int PAGE_SIZE = sysconf(_SC_PAGESIZE); //rozmiar strony 
    int w = 64
    int buffer_size = PAGE_SIZE * w;
    int address = 0; 

        __asm__ volatile (
            "movl $45, %%eax\n"
            "xorl %%ebx, %%ebx\n"
            "int $0x80\n"   //sys_brk - get the address of brk

            "movl %%eax, %%esi\n" //current position of brk 
            "movl %%eax, %%ebx\n"
            "addl %1, %%ebx\n" //4096(bytes)*64
            "movl $45, %%eax\n"
            "int $0x80\n" 
            "movl %%esi, %%eax\n"
            "movl %%esi, %0\n" //starting addres 
            "movl $42, (%%esi)\n"

            :"=g"(address)
            : "g"(buffer_size)
    );

//...