drm/i915: Restrict pagefault disabling to just around copy_from_user()
When handling execbuf relocations, we play a delicate dance with pagefault. We first try to access the user pages underneath our struct_mutex. However, if those pages were inside a GEM object, we may trigger a pagefault and deadlock as i915_gem_fault() tries to recursively acquire struct_mutex. Instead, we choose to disable pagefaulting around the copy_from_user whilst inside the struct_mutex and handle the EFAULT by falling back to a copy outside the struct_mutex. We however presumed that disabling pagefaults would be expensive. It is just an operation on the local current task. Cheap enough that we can restrict the disable/enable to the critical section around the copy, and so avoid having to handle the atomic sections within the relocation handling itself. v2: Just illustrate the broken error handling rather than argue why it is safer to ignore it, for now. Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@linux.intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/20161018120251.25043-4-chris@chris-wilson.co.uk
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