为什么mmap在32位armv7l上的4GB文件成功了？

Question

我从mmap(2)手册页和搜索结果中得到的印象mmap是,仅限于系统的可用地址空间,减去系统保留的地址空间.所以在32位armv7l上,我认为它大约是3GB =(4GB - 1GB).

但似乎我实际上可以mmap没有任何问题的5 GB文件:

int main(int argc, char** argv) {
        // stats
        char * path = argv[1];
        struct stat sb; 
        stat(path, &sb);
        std::cout << "File size: " << sb.st_size << std::endl;  

        // open
        int fd = open(path, O_RDONLY, S_IRWXU);
        std::cout << "File descriptor: " << fd << std::endl;
        int i;
        for (i =0; i<10; ++i){
                void *pa = mmap(
                        nullptr, sb.st_size, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, fd, 0);
                std::cout << "PA: " << pa  
                        << ", MAP_FAILED: " 
                        << (pa == MAP_FAILED) << ", Status: " 
                        << strerror(errno) << std::endl;
        }   
}

用-D_FILE_OFFSET_BITS=64旗子编译:

g++  -D_FILE_OFFSET_BITS=64 test.cc

结果产生:

File size: 5045966585
File descriptor: 3
PA: 0x89f80000, MAP_FAILED: 0, Status: Success
PA: 0x5d34a000, MAP_FAILED: 0, Status: Success
PA: 0x30714000, MAP_FAILED: 0, Status: Success
PA: 0x3ade000, MAP_FAILED: 0, Status: Success
PA: 0xffffffff, MAP_FAILED: 1, Status: Cannot allocate memory
PA: 0xffffffff, MAP_FAILED: 1, Status: Cannot allocate memory
PA: 0xffffffff, MAP_FAILED: 1, Status: Cannot allocate memory
PA: 0xffffffff, MAP_FAILED: 1, Status: Cannot allocate memory
PA: 0xffffffff, MAP_FAILED: 1, Status: Cannot allocate memory
PA: 0xffffffff, MAP_FAILED: 1, Status: Cannot allocate memory

从结果来看,mmap成功了4次才真正陷入困境.但它不应该成功,因为文件大约是5GB.

我的问题是:

1. 这种行为是期望的mmap吗？
2. 如果没有,我哪里做错了？

---

编辑:

使用物理地址扩展(PAE),32位系统可以添加超过2 ^ 32个字节(如果可用).

此CPU没有PAE支持

$> cat /proc/cpuinfo

Processor       : ARMv7 Processor rev 4 (v7l)
processor       : 0
BogoMIPS        : 1436.46

processor       : 1
BogoMIPS        : 1436.46

Features        : swp half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt 
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x0
CPU part        : 0xc07
CPU revision    : 4

Hardware        : sun7i
Revision        : 0000
Serial          : 09c11b9d52544848804857831651664b

Answer 1

PAE无关紧要.这不是关于访问大量物理内存.

问题是您的mmap函数采用32位值作为映射的大小.因此,您的64位大小会被截断,实际上您实际分配的虚拟内存不到1 GB.