linux-kernel - Gem GTT mmaps..

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Date:	Wed, 04 Feb 2009 23:32:00 +0100
From:	Thomas Hellström <thomas@...pmail.org>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
CC:	DRI <dri-devel@...ts.sourceforge.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Gem GTT mmaps..

Jesse,

I have some concerns about the GEM GTT mmap functionality.

First, a gem object pointer is copied to map->offset and then to the 
vma->vm_private_data without proper reference counting. This pointer is 
used in i915_gem_fault() to access the gem object. However if the gem 
object is destroyed and a process then tries to access data in a vma 
mapping the (now destroyed) object, it would dereference a stale pointer 
into kernel space? Shouldn't those pointers be reference counted, and to 
account for fork(), a vm open and close would be needed to  reference 
count corresponding pointers of newly created and destroyed vmas?

Second, the i915_gem_fault method  returns VM_FAULT_SIGBUS if 
vm_insert_pfn() fails with an -EBUSY. I think that's an error, since 
that would indicate that the pte was already populated by a racing thread.

/Thomas

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