linux-kernel - Re: [PATCH 1/6] drm/i915: Fix lock order reversal in GTT pwrite path.

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Date:	Thu, 26 Mar 2009 17:43:20 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	linux-kernel@...r.kernel.org, dri-devel@...ts.sourceforge.net,
	Eric Anholt <eric@...olt.net>
Subject: Re: [PATCH 1/6] drm/i915: Fix lock order reversal in GTT pwrite
 path.

On Wed, 25 Mar 2009 14:45:05 -0700
Eric Anholt <eric@...olt.net> wrote:

> Since the pagefault path determines that the lock order we use has to
> be mmap_sem -> struct_mutex, we can't allow page faults to occur
> while the struct_mutex is held.  To fix this in pwrite, we first try
> optimistically to see if we can copy from user without faulting.  If
> it fails, fall back to using get_user_pages to pin the user's memory,
> and map those pages atomically when copying it to the GPU.
> 
> Signed-off-by: Eric Anholt <eric@...olt.net>
> ---
> +	/* Pin the user pages containing the data.  We can't fault
> while
> +	 * holding the struct mutex, and all of the pwrite
> implementations
> +	 * want to hold it while dereferencing the user data.
> +	 */
> +	first_data_page = data_ptr / PAGE_SIZE;
> +	last_data_page = (data_ptr + args->size - 1) / PAGE_SIZE;
> +	num_pages = last_data_page - first_data_page + 1;
> +
> +	user_pages = kcalloc(num_pages, sizeof(struct page *),
> GFP_KERNEL);
> +	if (user_pages == NULL)
> +		return -ENOMEM;

If kmalloc limits us to a 128k allocation (and maybe less under
pressure), then we'll be limited to 128k/8 page pointers on 64 bit, or
64M per pwrite...  Is that ok?  Or do we need to handle multiple passes
here?

Looks good other than that.

Reviewed-by: Jesse Barnes <jbarnes@...tuousgeek.org>

-- 
Jesse Barnes, Intel Open Source Technology Center
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