---------- Forwarded message ---------- Date: Tue, 25 May 2004 15:40:56 -0700 (PDT) From: Ron Maeder <rlm@orionmulti.com> To: linux-mm@kvack.org Subject: mmap() > phys mem problem
I have a diskless x86 box running the 2.6.5rc3 kernel. I ran a program which mmap()'d a file that was larger than physical memory over NFS and then began to write values to it. The process grew until it was near the size of phys mem, and then grinded to a halt and other programs, including daemons, were exiting when they should have stayed running.
If I run the program on a system that has some swap space, it completes without any issue.
It seems as if the OS will not write any dirty pages back to the mmap()'d file, and then eventually runs out of memory.
Is this an "undocumented feature" or is this a linux error? I would expect pages of the mmap()'d file would get paged back to the original file. I know this won't be fast, but the performance is not an issue for this application.
Below is an example that reproduces the problem on a machine without swap. If I do an occasional synchronous msync(MS_SYNC) (compiling -DNEVER), the test case completes fine, while if I use an msync(MS_ASYNC) then other programs exit as if I did no msync().
Many thanks,
Ron --------------------------------------------------------------------------- #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <sys/mman.h> #include <sys/sysinfo.h>
#define MAX_UNSIGNED ((unsigned) (~0)) #define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)
unsigned
total_ram()
{
struct sysinfo info;
double my_total_ram; if (sysinfo(&info) != 0) {
perror("sysinfo");
exit(1);
}
my_total_ram = ((double) info.totalram * (double) info.mem_unit);
if (my_total_ram > (double) MAX_UNSIGNED) {
fprintf(stderr, "'my_total_ram' too large for 'unsigned' type.");
exit(1);
}
return((unsigned) my_total_ram);
}int
main()
{
unsigned i;
unsigned addr_size;
unsigned mem_size;
unsigned *mem;
char swap_filename[20] = "thrash_swap";
int swap_filedes; mem_size = total_ram();
mem_size -= (mem_size % sizeof(unsigned)); /* align to 'unsigned' size */
/* compute the size of the address for 'unsigned' memory accesses */
addr_size = ((mem_size / sizeof(unsigned)) - 1); (void) unlink(swap_filename);
if ((swap_filedes = open(swap_filename, O_RDWR | O_CREAT | O_TRUNC,
FILE_MODE)) == -1) {
perror("open: Can't open for writing");
exit(1);
}
/* Set size of swap file */
if (lseek(swap_filedes, (mem_size - 1), SEEK_SET) == (off_t) -1) {
perror("lseek");
exit(1);
}
if (write(swap_filedes, "", 1) != 1) {
perror("write");
exit(1);
}
if ((mem = (unsigned *) mmap(0, mem_size, PROT_READ | PROT_WRITE,
MAP_FILE | MAP_SHARED, swap_filedes, 0))
== (unsigned *) -1) {
perror("mmap");
exit(1);
}
/* for this example just dirty each page. */
for (i = 0; i < addr_size; i += 1024) {
mem[i] = 0;
if ((i & 0xfffff) == 0) {
#ifdef NEVER
if (msync(mem, mem_size, MS_SYNC) != 0) {
perror("msync");
exit(1);
}
#endif
printf(".");
fflush(stdout);
}
}
if (munmap(mem, mem_size) != 0) {
perror("munmap");
exit(1);
}
if (close(swap_filedes) != 0) {
perror("close");
exit(1);
}
if (unlink(swap_filename) != 0) {
perror("unlink");
exit(1);
}
printf("\n");
fflush(stdout);
return(0);
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