Gem GTT mmaps..

["Thomas Hellström" <thomas@shipmail.org>]

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