为什么 COW mmap 在大于 4GB 的（稀疏）文件上使用 ENOMEM 失败？

Question

当尝试使用写时复制语义（PROT_READ | PROT_WRITE 和 MAP_PRIVATE）映射 5GB 文件时，会在 2.6.26-2-amd64 Linux 内核上发生这种情况。映射小于 4GB 的文件或仅使用 PROT_READ 效果很好。这不是 这个问题；虚拟限制大小没有限制。

这是重现问题的代码（实际代码是 Boost.Interprocess）。

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>

#include <fcntl.h>
#include <unistd.h>

main()
{
        struct stat b;
        void *base;
        int fd = open("foo.bin", O_RDWR);

        fstat(fd, &b);
        base = mmap(0, b.st_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
        if (base == MAP_FAILED) {
                perror("mmap");
                return 1;
        }
        return 0;
}

发生的情况如下：

dd if=/dev/zero of=foo.bin bs=1M seek=5000 count=1
./test-mmap
mmap: Cannot allocate memory

这是相关的 strace （新编译的 4.5.20）输出，如 nos 所要求的。

open("foo.bin", O_RDWR)                 = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=5243928576, ...}) = 0
mmap(NULL, 5243928576, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = -1 ENOMEM (Cannot allocate memory)
dup(2)                                  = 4
[...]
write(4, "mmap: Cannot allocate memory\n", 29mmap: Cannot allocate memory
) = 29

Answer 1

尝试在 flags 字段中传递 MAP_NORESERVE，如下所示：

mmap(NULL, b.st_size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_NORESERVE, fd, 0);

您的交换内存和物理内存的组合可能小于请求的 5GB。

或者，您可以出于测试目的执行此操作，如果有效，您可以更改上面的代码：

# echo 0 > /proc/sys/vm/overcommit_memory

以下是手册页的相关摘录。

mmap(2):

   MAP_NORESERVE
          Do  not reserve swap space for this mapping.  When swap space is
          reserved, one has the guarantee that it is  possible  to  modify
          the  mapping.   When  swap  space  is not reserved one might get
          SIGSEGV upon a write if no physical memory  is  available.   See
          also  the  discussion of the file /proc/sys/vm/overcommit_memory
          in proc(5).  In kernels before 2.6, this flag  only  had  effect
          for private writable mappings.

过程(5):

   /proc/sys/vm/overcommit_memory
          This file contains the kernel virtual  memory  accounting  mode.
          Values are:

                 0: heuristic overcommit (this is the default)
                 1: always overcommit, never check
                 2: always check, never overcommit

          In  mode 0, calls of mmap(2) with MAP_NORESERVE are not checked,
          and the default check is very weak, leading to the risk of  get‐
          ting a process "OOM-killed".  Under Linux 2.4 any non-zero value
          implies mode 1.  In mode 2  (available  since  Linux  2.6),  the
          total  virtual  address  space on the system is limited to (SS +
          RAM*(r/100)), where SS is the size of the swap space, and RAM is
          the  size  of  the physical memory, and r is the contents of the
          file /proc/sys/vm/overcommit_ratio.

Answer 2

从评论中引用您的内存、交换大小和过量使用设置：

MemTotal: 4063428 kB SwapTotal: 514072 kB
$ cat /proc/sys/vm/overcommit_memory
0
$ cat /proc/sys/vm/overcommit_ratio 
50

将 overcommit_memory 设置为 0（“启发式过量使用”）时，您无法创建大于当前可用内存的私有可写映射，并且交换总数 - 显然，由于您只有 4.5GB 内存 + 交换，所以这永远不可能是真的。

您的选择是使用 MAP_NORESERVE （如 Matt Joiner 建议），如果您确定您永远不会弄脏（写入）映射中的页面多于可用内存和交换空间为了;或者显着增加交换空间的大小。