为什么在32位armv7l上成功映射4GB文件? [英] Why mmap a 4GB file on 32-bit armv7l succeeded?

问题描述

我从mmap(2)手册页和搜索结果中得到的印象是，mmap仅限于系统的可用地址空间，减去系统保留的地址空间.因此，在32位armv7l上，我假设它约为3GB =(4GB-1GB).

I had the impression from the mmap(2) man page and search results, that mmap is only limited to system's available address spaces, minus the system reserved address spaces. So on 32-bit armv7l, I assume it's around 3GB = (4GB - 1GB).

但是看来我实际上可以mmap一个5 GB的文件而没有任何问题:

But it seemed like I could actually mmap a 5 GB file without any problem:

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;
        }   
}

使用-D_FILE_OFFSET_BITS=64标志进行编译:

g++  -D_FILE_OFFSET_BITS=64 test.cc

结果如下:

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在经历真正的麻烦之前已经成功了4次.但这不应该成功，因为文件约为5GB.

From the results, mmap succeeded for 4 times before going into real troubles. But it shouldn't have been succeeded since the file is ~5GB.

我的问题是:

1. mmap会发生这种情况吗?
2. 如果没有，我在哪里做错了?

1. Is this behavior expected for mmap?
2. If not, where did I do wrong?

---

---

如果有物理地址扩展(PAE)，则32位系统可以添加的地址远远超过2 ^ 32字节.

With physical addres extension (PAE) 32-bit systems can addres much more than 2^32 bytes, if that is available.

此CPU不支持PAE

There's no PAE support for this CPU

$> 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

推荐答案

PAE不相关.这与访问大量物理内存无关.

PAE is irrelevant. This is not about accessing large amounts of physical memory.

问题是您的mmap函数采用32位值表示映射大小.因此，您的64位大小会被截断，实际上您分配的虚拟内存不到1 GB.

The problem is that your mmap function takes a 32-bit value for the size of the mapping. So your 64-bit size gets truncated and you're actually allocating less than 1 GB of virtual memory.

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