Segmentation error due to initialize () function when resizing a huge page selection

Question

I encounter a segmentation error in my program when I play with the size of the memory that I want to allocate with a huge page, i.e. when I define LENGTH = 4 * 1024, a seg error occurs. When I define 4 * 1024 * 1024, there is no seg error. What is the reason for this?

Code below:

#define _POSIX_C_SOURCE 199309 #define _GNU_SOURCE #include <sched.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/mman.h> #include <fcntl.h> #include <time.h> #include <string.h> #include <errno.h> #define PROTECTION (PROT_READ | PROT_WRITE) #define LENGTH (4*1024) //#define LENGTH (4*1024*1024) #define LINE_SIZE 64 #define ASSOC 16 #define CACHE_SIZE (4*1024*1024) #define WAY_SIZE (CACHE_SIZE/ASSOC) #ifndef MAP_HUGETLB #define MAP_HUGETLB 0x40000 #endif #define ADDR (void *) (0x0UL) #define FLAGS (MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB) int main (int argc, char *argv[]){ ... // allocate a buffer with the same size as the LLC using huge pages buf = mmap(ADDR, LENGTH, PROTECTION, FLAGS, 0, 0); if (buf == MAP_FAILED) { perror("mmap"); exit(1); } set_S = atoi(argv[1]); set_M = atoi(argv[2]); printf("test 0\n"); initialize(set_S); printf("test 1\n"); initialize(set_M); printf("test 2\n"); head_S = &buf[set_S*LINE_SIZE]; printf("test 3\n"); head_M = &buf[set_M*LINE_SIZE]; ... } 

Please note that on my computer the huge page is not enabled by default, so I turned it on as root:

 echo 20 > /proc/sys/vm/nr_hugepages 

This is a huge page related to the information in / proc / meminfo after I include a huge page:

 HugePages_Total: 20 HugePages_Free: 20 HugePages_Rsvd: 0 HugePages_Surp: 0 Hugepagesize: 2048 kB 

Program output:

 $./a.out 1 1 test 0 Segmentation fault (core dumped) 

$ uname output:

 Linux mymachine 3.13.0-43-generic #72-Ubuntu SMP Mon Dec 8 19:35:06 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux 

EDIT 1: Add initialization function:

 void initialize(int set){ int j, k; char ** ptr1, ** ptr2; char * tmp; // re-initialize pointer array of size "size" // implementation of Sattolo random cyclic permutation for (j=0; j<ASSOC; j++){ ptr1 = (char **)&buf[set*LINE_SIZE+j*WAY_SIZE]; *ptr1 = (char*)ptr1; } // permute each set for (j=ASSOC-1; j>=1; j--){ k = rand()%j; ptr1 = (char **)&buf[set*LINE_SIZE+j*WAY_SIZE]; ptr2 = (char **)&buf[set*LINE_SIZE+k*WAY_SIZE]; tmp = *ptr1; *ptr1 = *ptr2; *ptr2 = tmp; } } 

Answer 1

When accessing set*LINE_SIZE+(ASSOC-1)*WAY_SIZE = set*64 + CACHE_SIZE - WAY_SIZE = set*64 + 4*1024*1024 - WAY_SIZE , which is greater than the length of mmap LENGTH = 4*1024 in the initialize loop initialize you access memory from the bound . This is the reason you had a segment error while determining LENGTH = 4 * 1024.