drm/i915: Fix lock order reversal in GTT pwrite path.
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@anholt.net> Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
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