Re: [PATCH][RFC][0/4] InfiniBand userspace verbs implementation

From: Timur Tabi
Date: Mon Apr 25 2005 - 17:24:00 EST

Andrew Morton wrote:

The way we expect get_user_pages() to be used is that the kernel will use
get_user_pages() once per application I/O request.

Are you saying that RDMA clients will semi-permanently own pages which were
pinned by get_user_pages()? That those pages will be used for multiple
separate I/O operations?

Yes, absolutely!

The memory buffer is allocated by the process (usually just via malloc) and registed/pinned by the driver. It then stays pinned for the life of the process (typically).

If so, then that's a significant design departure and it would be good to
hear why it is necessary.

That's just how RMDA works. Once the memory is pinned, if the app wants to send data to another node, it does two things:

1) Puts the data into its buffer
2) Sends a "work request" to the driver with (among other things) the offset and length of the data.

This is a time-critical operation. It must occurs as fast as possible, which means the memory must have already been pinned.

Timur Tabi
Staff Software Engineer

One thing a Southern boy will never say is,
"I don't think duct tape will fix it."
-- Ed Smylie, NASA engineer for Apollo 13
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