bdflush - 启动，刷新，或调缓冲区脏刷新守护
int bdflush(int func, long *address); int bdflush(int func, long data);
bdflush() starts, flushes, or tunes the buffer-dirty-flush daemon. Only a privileged process (one with the CAP_SYS_ADMIN capability) may call bdflush().
If func is negative or 0, and no daemon has been started, then bdflush() enters the daemon code and never returns.
If func is 1, some dirty buffers are written to disk.
If func is 2 or more and is even (low bit is 0), then address is the address of a long word, and the tuning parameter numbered (func-2)/2 is returned to the caller in that address.
If func is 3 or more and is odd (low bit is 1), then data is a long word, and the kernel sets tuning parameter numbered (func-3)/2 to that value.
The set of parameters, their values, and their legal ranges are defined in the kernel source file fs/buffer.c.
If func is negative or 0 and the daemon successfully starts, bdflush() never returns. Otherwise, the return value is 0 on success and -1 on failure, with errno set to indicate the error.
|EBUSY||An attempt was made to enter the daemon code after another process has already entered.|
|EFAULT||address points outside your accessible address space.|
|EINVAL||An attempt was made to read or write an invalid parameter number, or to write an invalid value to a parameter.|
|EPERM||Caller does not have the CAP_SYS_ADMIN capability.|
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