Date: Mon, 2 Mar 2015 15:49:56 -0800 From: Adrian Chadd <adrian@freebsd.org> To: "K. Macy" <kmacy@freebsd.org> Cc: freebsd-current <freebsd-current@freebsd.org>, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org> Subject: Re: Doing zero-copy stuff in drivers, or "is vm_fault_quick_hold_pages() enough" ? Message-ID: <CAJ-VmonpLG4BTmJvnzOe7HJMaZ3Z8iVzwG62xZkAtCHb=94AQA@mail.gmail.com> In-Reply-To: <CAHM0Q_PnW%2BmhSU4SeAuwBJvvAx6G3fsVDNAN6Fpt1JUFn5_ysQ@mail.gmail.com> References: <CAJ-Vmom87%2BC0dT5gj8YVGH-QjqDHU3otKeb9XaDSnwwzrY5hPQ@mail.gmail.com> <CAHM0Q_PnW%2BmhSU4SeAuwBJvvAx6G3fsVDNAN6Fpt1JUFn5_ysQ@mail.gmail.com>
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On 2 March 2015 at 15:05, K. Macy <kmacy@freebsd.org> wrote:
> On Mon, Mar 2, 2015 at 12:42 PM, Adrian Chadd <adrian@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.
Well, you can look at what's going on in the vchiq code. gonzo made it
an explicit wire in order to avoid issues.
> 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").
> */
What's the definition of "very temporary holding" ? What's the
behavioural difference?
-adrian
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