Greetings.
When working with large files, such as git has to when dealing with
packs, I noticed that mmap fails when the requested length of the
mapping is approaching the size of physical memory (no swap configured).
It is impossible to — even just read-only — call mmap on a file or
anonymous object and requesting a region despite 64-bit architectures
having the address space for it.
This looks like a bug to me.
I presume that mapping a, say 2 GB file on a system with 2 GB RAM does
not require 2 GB just for the PTEs.
Observed on 2.6.33, 2.6.36.
---
$ ls -log devqa.dsk
-rw-r----- 1 8589934592 Nov 30 02:58 devqa.dsk
$ ./mmap devqa.dsk $[1048576*2048]
sizeof(size_t) = 8
devqa.dsk for 2147483648
mmap: Cannot allocate memory
0xffffffffffffffff
$ free
total used free shared buffers cached
Mem: 2051340 113076 1938264 0 10928 67776
-/+ buffers/cache: 34372 2016968
Swap: 0 0 0
/proc/meminfo:
MemTotal: 2051340 kB
MemFree: 1938264 kB
Buffers: 10936 kB
Cached: 58848 kB
SwapCached: 0 kB
Active: 35560 kB
Inactive: 45696 kB
Active(anon): 11828 kB
Inactive(anon): 0 kB
Active(file): 23732 kB
Inactive(file): 45696 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 8 kB
Writeback: 0 kB
AnonPages: 11468 kB
Mapped: 8964 kB
Shmem: 360 kB
Slab: 16976 kB
SReclaimable: 8932 kB
SUnreclaim: 8044 kB
KernelStack: 1032 kB
PageTables: 2120 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 1025668 kB
Committed_AS: 82680 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 2672 kB
VmallocChunk: 34359730812 kB
DirectMap4k: 8128 kB
DirectMap2M: 2088960 kB
---
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
int main(int argc, const char **argv)
{
int fd = open(argv[1], O_RDONLY);
if (fd < 0)
abort();
size_t len = strtoull(argv[2], NULL, 0);
printf("sizeof(size_t) = %zu\n", sizeof(size_t));
if (sizeof(size_t) < 8)
abort();
printf("%s for %zu\n", argv[1], len);
void *x = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
if (x == (void *)-1)
perror("mmap");
printf("%p\n", x);
return 0;
}