drbd: always set BLK_FEAT_STABLE_WRITES

DRBD requires stable pages because it may read the same bio data
multiple times for local disk I/O and network transmission, and in
some cases for calculating checksums.

The BLK_FEAT_STABLE_WRITES flag is set when the device is first
created, but blk_set_stacking_limits() clears it whenever a
backing device is attached. In some cases the flag may be
inherited from the backing device, but we want it to be enabled
at all times.

Unconditionally re-enable BLK_FEAT_STABLE_WRITES in
drbd_reconsider_queue_parameters() after the queue parameter
negotiations.

Also, document why we want this flag enabled in the first place.

Fixes: 1a02f3a ("block: move the stable_writes flag to queue_limits")
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: chrboe

drivers/block/drbd/drbd_main.c

@@ -2659,9 +2659,6 @@ enum drbd_ret_code drbd_create_device(struct drbd_config_context *adm_ctx, unsig
 		 * connect.
 		 */
 		.max_hw_sectors		= DRBD_MAX_BIO_SIZE_SAFE >> 8,
-		.features		= BLK_FEAT_WRITE_CACHE | BLK_FEAT_FUA |
-					  BLK_FEAT_ROTATIONAL |
-					  BLK_FEAT_STABLE_WRITES,
 	};
 
 	device = minor_to_device(minor);

drivers/block/drbd/drbd_nl.c

@@ -1296,6 +1296,8 @@ void drbd_reconsider_queue_parameters(struct drbd_device *device,
 		lim.max_segments = drbd_backing_dev_max_segments(device);
 	} else {
 		lim.max_segments = BLK_MAX_SEGMENTS;
+		lim.features = BLK_FEAT_WRITE_CACHE | BLK_FEAT_FUA |
+			       BLK_FEAT_ROTATIONAL | BLK_FEAT_STABLE_WRITES;
 	}
 
 	lim.max_hw_sectors = new >> SECTOR_SHIFT;
@@ -1318,8 +1320,24 @@ void drbd_reconsider_queue_parameters(struct drbd_device *device,
 		lim.max_hw_discard_sectors = 0;
 	}
 
-	if (bdev)
+	if (bdev) {
 		blk_stack_limits(&lim, &b->limits, 0);
+		/*
+		 * blk_set_stacking_limits() cleared the features, and
+		 * blk_stack_limits() may or may not have inherited
+		 * BLK_FEAT_STABLE_WRITES from the backing device.
+		 *
+		 * DRBD always requires stable writes because:
+		 * 1. The same bio data is read for both local disk I/O and
+		 *    network transmission. If the page changes mid-flight,
+		 *    the local and remote copies could diverge.
+		 * 2. When data integrity is enabled, DRBD calculates a
+		 *    checksum before sending the data. If the page changes
+		 *    between checksum calculation and transmission, the
+		 *    receiver will detect a checksum mismatch.
+		 */
+		lim.features |= BLK_FEAT_STABLE_WRITES;
+	}
 
 	/*
 	 * If we can handle "zeroes" efficiently on the protocol, we want to do