From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH 2/2] drm/i915: properly prefault for
 pread/pwrite
Date: Mon, 24 Oct 2011 00:11:57 +0200
Message-ID: <20111023221157.GH2953@phenom.ffwll.local>
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To: Keith Packard <keithp@keithp.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>, intel-gfx <intel-gfx@lists.freedesktop.org>, stable@kernel.org
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On Sun, Oct 23, 2011 at 12:23:47PM -0700, Keith Packard wrote:
> On Sun, 23 Oct 2011 12:18:30 +0200, Daniel Vetter <daniel@ffwll.ch> wrote:
> > Hi Keith,
> > 
> > This patch isn't in your -next pull. This papers over a spurious -EFAULT
> > in the pwrite/pread paths that actually gets hit in the wild. The real fix
> > in the form of a almost complete rewrite of the pwrite/pread paths won't
> > be ready for 3.2.
> 
> We had several comments wondering whether writing zeros was OK as this
> occurs before some potential error returns that should leave the buffer
> unmodified. I didn't have a better suggestion, but that seems pretty
> sketchy to me.

This patch only fixes things up so that we prefault the entire page range
and not just the first PAGE_SIZE bytes (i.e. at most 2 pages). So I don't
see the risk of extending the current behaviour to all pages. Userspace
can already see these zero writes, but only when doing something stupid.
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48