Re: [PATCH 1/2] drm/i915: Prefault all pages for pread and pwrite

From: Ben Widawsky <ben@bwidawsk.net>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 1/2] drm/i915: Prefault all pages for pread and pwrite
Date: Sun, 10 Jul 2011 01:40:31 +0000	[thread overview]
Message-ID: <20110710014031.GA8410@cloud01> (raw)
In-Reply-To: <1310200731-18086-1-git-send-email-chris@chris-wilson.co.uk>

On Sat, Jul 09, 2011 at 09:38:50AM +0100, Chris Wilson wrote:
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 4fc9738..2fce620 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -503,6 +503,19 @@ out:
>  	return ret;
>  }
>  
> +static int prefault_writeable(unsigned long uaddr, unsigned long len)
> +{
> +	int ret = 0;
> +
> +	len += uaddr;
> +	while (uaddr < len) {
> +		ret |= __put_user(0, (char __user *)uaddr);
> +		uaddr += 4096;
> +	}
> +
> +	return ret ? -EFAULT : 0;
> +}
> +
>  /**
>   * Reads data from the object referenced by handle.
>   *

What's the reason for not breaking out of the loop on the first failing
fault? It seems like you could incur a bunch of latency for a call which
you know will ending up failin anyway. Although TBH I'm not clear how
it could actually fail if you've called access_ok().

Also you should probably use PAGE_SIZE instead of 4096.

Ben