Doing a zero-copy move of data from a Linux kernel buffer to hard disk

Question

am trying to move data from a buffer in kernel space into the hard disk without having to incur any additional copies from kernel buffer to user buffers or any other kernel buffers. Any ideas/suggestions would be most helpful.

The use case is basically a demux driver which collects data into a demux buffer in kernel space and this buffer has to be emptied periodically by copying the contents into a FUSE-based partition on the disk. As the buffer gets full, a user process is signalled which then determines the sector numbers on the disk the contents need to be copied to.

I was hoping to mmap the above demux kernel buffer into user address space and issue a write system call to the raw partition device. But from what I can see, the this data is being cached by the kernel on its way to the Hard Disk driver. And so I am assuming that involves additional copies by the linux kernel.

At this point I am wondering if there is any other mechansim to do this without involving additional copies by the kernel. I realize this is an unsual usage scenario for non-embedded environments, but I would appreciate any feedback on possible options.

BTW - I have tried using O_DIRECT when opening the raw partition, but the subsequent write call fails if the buffer being passed is the mmapped buffer.

Thanx!

Answer 1

You need to expose your demux buffer as a file descriptor (presumably, if you're using mmap() then you're already doing this - great!).

On the kernel side, you then need to implement the splice_read member of struct file_operations.

On the userspace side, create a pipe(), then use splice() twice - once to move the data from the demux file descriptor into the pipe, and a second time to move the data from the pipe to the disk file. Use the SPLICE_F_MOVE flag.

As documented in the splice() man page, it will avoid actual copies where it can, by copying references to pages of kernel memory rather than the pages themselves.