Date: Mon, 25 Apr 2005 15:32:56 -0700
From: Andrew Morton <akpm@osdl.org>
To: Timur Tabi <timur.tabi@ammasso.com>
Cc: roland@topspin.com, hch@infradead.org, hozer@hozed.org,
       linux-kernel@vger.kernel.org, openib-general@openib.org
Subject: Re: [PATCH][RFC][0/4] InfiniBand userspace verbs implementation
Message-Id: <20050425153256.3850ee0a.akpm@osdl.org>
In-Reply-To: <426D6D68.6040504@ammasso.com>
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Timur Tabi <timur.tabi@ammasso.com> wrote:
>
> 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).

ug.  What stops the memory from leaking if the process exits?

I hope this is a privileged operation?

> > 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.

It would be better to obtain this memory via a mmap() of some special
device node, so we can perform appropriate permission checking and clean
everything up on unclean application exit.

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