drm/i915: make i915_gem_mmap_ioctl wait for mmap_sem killable

i915_gem_mmap_ioctl relies on mmap_sem for write.  If the waiting task
gets killed by the oom killer it would block oom_reaper from
asynchronous address space reclaim and reduce the chances of timely OOM
resolving.  Wait for the lock in the killable mode and return with EINTR
if the task got killed while waiting.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/i915_gem.c

@@ -1754,7 +1754,10 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 		struct mm_struct *mm = current->mm;
 		struct vm_area_struct *vma;
 
-		down_write(&mm->mmap_sem);
+		if (down_write_killable(&mm->mmap_sem)) {
+			drm_gem_object_unreference_unlocked(obj);
+			return -EINTR;
+		}
 		vma = find_vma(mm, addr);
 		if (vma)
 			vma->vm_page_prot =