Doing zero-copy stuff in drivers, or "is vm_fault_quick_hold_pages() enough" ?

[K. Macy]

On Mon, Mar 2, 2015 at 12:42 PM, Adrian Chadd <adrian at freebsd.org> wrote:
> Hi,
>
> gonzo@ committed a fix (r278615) to the videocore driver for the
> raspberry pi. The fix involved doing an explicit wire of pages that
> were about to be passed down to the hardware to send to the
> videobuffer hardware.
>
> It turns out that doing vm_fault_quick_hold_pages() wasn't enough -
> the pages weren't being wired, and they may disappear before the
> hardware gets to them.


Then your code is buggy or you've hit a bug in the VM. Holding a page
is a short-term wiring.

Right above vm_page_hold():
/*
 * Keep page from being freed by the page daemon
 * much of the same effect as wiring, except much lower
 * overhead and should be used only for *very* temporary
 * holding ("wiring").
 */


> I looked at vmapbuf() and how it uses vm_fault_quick_hold_pages(), but
> I can't find anything that wires the pages down before it hands the
> addresses to the hardware.
>
> So, am I missing something about how/where that's done?
Yes.

> Thanks,
>
>
> -adrian
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