drm/i915: Improve behaviour under memory pressure

Due to the necessity of having to take the struct_mutex, the i915
shrinker can not free the inactive lists if we fail to allocate memory
whilst processing a batch buffer, triggering an OOM and an ENOMEM that
is reported back to userspace. In order to fare better under such
circumstances we need to manually retry a failed allocation after
evicting inactive buffers.

To do so involves 3 steps:
1. Marking the backing shm pages as NORETRY.
2. Updating the get_pages() callers to evict something on failure and then
   retry.
3. Revamping the evict something logic to be smarter about the required
   buffer size and prefer to use volatile or clean inactive pages.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/drm_gem.c

@@ -142,6 +142,19 @@ drm_gem_object_alloc(struct drm_device *dev, size_t size)
 	if (IS_ERR(obj->filp))
 		goto free;
 
+	/* Basically we want to disable the OOM killer and handle ENOMEM
+	 * ourselves by sacrificing pages from cached buffers.
+	 * XXX shmem_file_[gs]et_gfp_mask()
+	 */
+	mapping_set_gfp_mask(obj->filp->f_path.dentry->d_inode->i_mapping,
+			     GFP_HIGHUSER |
+			     __GFP_COLD |
+			     __GFP_FS |
+			     __GFP_RECLAIMABLE |
+			     __GFP_NORETRY |
+			     __GFP_NOWARN |
+			     __GFP_NOMEMALLOC);
+
 	kref_init(&obj->refcount);
 	kref_init(&obj->handlecount);
 	obj->size = size;