Documentation/vm/hugepage-shm.c

   1 /*
   2  * hugepage-shm:
   3  *
   4  * Example of using huge page memory in a user application using Sys V shared
   5  * memory system calls.  In this example the app is requesting 256MB of
   6  * memory that is backed by huge pages.  The application uses the flag
   7  * SHM_HUGETLB in the shmget system call to inform the kernel that it is
   8  * requesting huge pages.
   9  *
  10  * For the ia64 architecture, the Linux kernel reserves Region number 4 for
  11  * huge pages.  That means that if one requires a fixed address, a huge page
  12  * aligned address starting with 0x800000... will be required.  If a fixed
  13  * address is not required, the kernel will select an address in the proper
  14  * range.
  15  * Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
  16  *
  17  * Note: The default shared memory limit is quite low on many kernels,
  18  * you may need to increase it via:
  19  *
  20  * echo 268435456 > /proc/sys/kernel/shmmax
  21  *
  22  * This will increase the maximum size per shared memory segment to 256MB.
  23  * The other limit that you will hit eventually is shmall which is the
  24  * total amount of shared memory in pages. To set it to 16GB on a system
  25  * with a 4kB pagesize do:
  26  *
  27  * echo 4194304 > /proc/sys/kernel/shmall
  28  */
  29
  30 #include <stdlib.h>
  31 #include <stdio.h>
  32 #include <sys/types.h>
  33 #include <sys/ipc.h>
  34 #include <sys/shm.h>
  35 #include <sys/mman.h>
  36
  37 #ifndef SHM_HUGETLB
  38 #define SHM_HUGETLB 04000
  39 #endif
  40
  41 #define LENGTH (256UL*1024*1024)
  42
  43 #define dprintf(x)  printf(x)
  44
  45 /* Only ia64 requires this */
  46 #ifdef __ia64__
  47 #define ADDR (void *)(0x8000000000000000UL)
  48 #define SHMAT_FLAGS (SHM_RND)
  49 #else
  50 #define ADDR (void *)(0x0UL)
  51 #define SHMAT_FLAGS (0)
  52 #endif
  53
  54 int main(void)
  55 {
  56         int shmid;
  57         unsigned long i;
  58         char *shmaddr;
  59
  60         if ((shmid = shmget(2, LENGTH,
  61                             SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W)) < 0) {
  62                 perror("shmget");
  63                 exit(1);
  64         }
  65         printf("shmid: 0x%x\n", shmid);
  66
  67         shmaddr = shmat(shmid, ADDR, SHMAT_FLAGS);
  68         if (shmaddr == (char *)-1) {
  69                 perror("Shared memory attach failure");
  70                 shmctl(shmid, IPC_RMID, NULL);
  71                 exit(2);
  72         }
  73         printf("shmaddr: %p\n", shmaddr);
  74
  75         dprintf("Starting the writes:\n");
  76         for (i = 0; i < LENGTH; i++) {
  77                 shmaddr[i] = (char)(i);
  78                 if (!(i % (1024 * 1024)))
  79                         dprintf(".");
  80         }
  81         dprintf("\n");
  82
  83         dprintf("Starting the Check...");
  84         for (i = 0; i < LENGTH; i++)
  85                 if (shmaddr[i] != (char)i)
  86                         printf("\nIndex %lu mismatched\n", i);
  87         dprintf("Done.\n");
  88
  89         if (shmdt((const void *)shmaddr) != 0) {
  90                 perror("Detach failure");
  91                 shmctl(shmid, IPC_RMID, NULL);
  92                 exit(3);
  93         }
  94
  95         shmctl(shmid, IPC_RMID, NULL);
  96
  97         return 0;
  98 }