Running out of memory in 1 easy step

[Wichert Akkerman <wichert@soil.nl>]

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I have a small test program that consistently can't allocate more
memory using mmap after 458878 allocations, no matter how much memory
I allocate per call (tried with 8, 80, 800 and 4000 bytes per call):
mmap returns ENOMEM. The machine has plenty memory available (2Gb
and no other processes are running except standard daemons) so there
should be enough memory.

Some added printk statements in mm/mmap.c do_mmap_pgoff() revealed
this happens at the following bit of code:

        /* Obtain the address to map to. we verify (or select) it and ensure
         * that it represents a valid section of the address space.
         */
        if (flags & MAP_FIXED) {
                if (addr & ~PAGE_MASK)
                        return -EINVAL;
        } else {
                addr = get_unmapped_area(addr, len);
                if (!addr) {
                        printk("do_mmap_pgoff: cannot allocate unmapped memory, returning ENOMEM\n");
                        return -ENOMEM;
                }
        }

Silly test program is attached (I didn't write it, so don't punish me for
the ugly code :).

Wichert.

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#include <unistd.h>
#include <sys/mman.h>
  
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
int main(){
  int k;
   unsigned int m,p;
   const int mem=800000000;
   int len;
   double **dptr;

   m=sizeof(double)+sizeof(double*);
   len=mem/m;
printf("Will allocate %d bytes in %d blocks\n", mem, len);
   dptr = (double **) malloc(sizeof(double*)*len) ;
   for(k=0; k<len; k++) {
     dptr[k] = (double *) mmap(NULL, 500*sizeof(double), PROT_READ|PROT_WRITE, (MAP_PRIVATE|MAP_ANONYMOUS), -1, 0);
     if(dptr[k]==MAP_FAILED) {
	printf("Allocation error: %s\n", strerror(errno));
       printf("k: %d\n",k);
       printf("Alloc Null\n");
getchar();
       exit(-1);
     }
     *(dptr[k]) = rand();
   }
   for(k=len-1; k>=0; k--) {
     printf("value: %d",*(dptr[k]));
   }
   fprintf(stderr,"k=%d mem=%d MB\n",k,mem/1000000);
   exit (0);
 }