more comments about gfp flags, two GFP_KERNEL -> GFP_NOIO

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_actlog.c

@@ -588,6 +588,7 @@ static int atodb_prepare_unless_covered(struct drbd_conf *mdev,
 	}
 	/* bios[i] == NULL, the next not yet used slot */
 
+	/* GFP_KERNEL, we are not in the write-out path */
 	bio = bio_alloc(GFP_KERNEL, 1);
 	if (bio == NULL)
 		return -ENOMEM;
@@ -661,6 +662,7 @@ void drbd_al_to_on_disk_bm(struct drbd_conf *mdev)
 
 	nr_elements = mdev->act_log->nr_elements;
 
+	/* GFP_KERNEL, we are not in anyones write-out path */
 	bios = kzalloc(sizeof(struct bio *) * nr_elements, GFP_KERNEL);
 	if (!bios)
 		goto submit_one_by_one;

drivers/block/drbd/drbd_main.c

@@ -2998,6 +2998,7 @@ struct drbd_conf *drbd_new_device(unsigned int minor)
 	struct gendisk *disk;
 	struct request_queue *q;
 
+	/* GFP_KERNEL, we are outside of all write-out paths */
 	mdev = kzalloc(sizeof(struct drbd_conf), GFP_KERNEL);
 	if (!mdev)
 		return NULL;

drivers/block/drbd/drbd_receiver.c

@@ -1299,7 +1299,10 @@ read_in_block(struct drbd_conf *mdev, u64 id, sector_t sector, int data_size) __
 	ERR_IF(data_size &  0x1ff) return NULL;
 	ERR_IF(data_size >  DRBD_MAX_SEGMENT_SIZE) return NULL;
 
-	e = drbd_alloc_ee(mdev, id, sector, data_size, GFP_KERNEL);
+	/* GFP_NOIO, because we must not cause arbitrary write-out: in a DRBD
+	 * "criss-cross" setup, that might cause write-out on some other DRBD,
+	 * which in turn might block on the other node at this very place.  */
+	e = drbd_alloc_ee(mdev, id, sector, data_size, GFP_NOIO);
 	if (!e)
 		return NULL;
 	bio = e->private_bio;
@@ -1966,7 +1969,10 @@ static int receive_DataRequest(struct drbd_conf *mdev, struct p_header *h)
 		return TRUE;
 	}
 
-	e = drbd_alloc_ee(mdev, p->block_id, sector, size, GFP_KERNEL);
+	/* GFP_NOIO, because we must not cause arbitrary write-out: in a DRBD
+	 * "criss-cross" setup, that might cause write-out on some other DRBD,
+	 * which in turn might block on the other node at this very place.  */
+	e = drbd_alloc_ee(mdev, p->block_id, sector, size, GFP_NOIO);
 	if (!e) {
 		put_ldev(mdev);
 		return FALSE;

drivers/block/drbd/drbd_worker.c

@@ -367,6 +367,8 @@ static int read_for_csum(struct drbd_conf *mdev, sector_t sector, int size)
 	if (FAULT_ACTIVE(mdev, DRBD_FAULT_AL_EE))
 		return 2;
 
+	/* GFP_TRY, because if there is no memory available right now, this may
+	 * be rescheduled for later. It is "only" background resync, after all. */
 	e = drbd_alloc_ee(mdev, DRBD_MAGIC+0xbeef, sector, size, GFP_TRY);
 	if (!e) {
 		put_ldev(mdev);