linux - 直接 IO 的 mmap : bad address?

Question

我使用匿名 mmap 分配了一些内存:

buff->addr = mmap(NULL, length, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS -1, 0);
fprintf(stderr, "allocated buffer: %p, %lu\n", buff->addr, (unsigned long)length);

然后我用 O_DIRECT 打开的 fd 写信给它:

int fd = open(name, O_CREAT | O_TRUNC | O_WRONLY | O_DIRECT, 00300);
if(fd == -1)
{
    perror("failed to open for write");
    return;
}

fprintf(stderr, "writing to address: %p size: %lu\n", buff.addr, buff.length);
if(write(fd, buff.addr, buff.length) != size)
{
    perror("failed to write");
}

但得到错误的地址错误:

allocated buffer: 0x7f096ddff000, 512000
writing to address: 0x7f096ddff000 size: 512000
failed to write: Bad address

我觉得地址不错。

如果我在 .data 中分配缓冲区并对齐到 512，则写入有效，因此 open() 和 write() 没问题，我拥有所有权限等。

怎么了？除 512 对齐外，直接 IO 是否还有其他要求？

更新:

例如，如果我为此更改 mmap:

char buffer[ 1024000 ];
for(buff->addr = buffer; ((unsigned long)buff->addr % 512) != 0; buff->addr = buff->addr + 1);

有效:

allocated buffer: 0x500e00, 512000
writing to address: 0x500e00 size: 512000

Answer 1

PROT_NONE Pages may not be accessed.

你至少不应该将 PROT_READ 作为 mmap 的 3. 参数吗？为了在这种情况下有用，您可能需要 PROT_READ|PROT_WRITE