Re: [PATCH 2/2] drm/i915: Disable page-faults around the fast pwrite/pread paths

From: Keith Packard <keithp@keithp.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 2/2] drm/i915: Disable page-faults around the fast pwrite/pread paths
Date: Sat, 09 Jul 2011 14:06:23 -0700	[thread overview]
Message-ID: <yunoc13f9kg.fsf@aiko.keithp.com> (raw)
In-Reply-To: <aefc95$ohq3u@orsmga001.jf.intel.com>

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On Sat, 09 Jul 2011 21:50:26 +0100, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> 
> I think would do, find_vma() is not necessary cheap though, and there are a
> couple of optimisations that we haven't done for pwrite/pread yet to speed
> up the transition to the slow path.

Yeah, find_vma is a rb tree walk over the whole address space. Yikes!
And, of course, we'd actually need to walk over the whole mapping in
case the application managed to walk from non-GTT space into GTT space.

> The current code generates lockdep OOPSes and inconsistently applies
> pagefault_disable along some paths, in particular for 32-bit kernels,
> but not others. And the abuse is permitted through the OpenGL
> specification, I believe. The offending app is just doing
> glBufferData(glMapBuffer()), iiuc;

Sure, it's permitted, so ideally we'd detect this abuse and fall back to
the slow path, but we need a cheap check which takes the slow path,
perhaps pessimistically.

-- 
keith.packard@intel.com

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