drbd: Improve compatibility with drbd's older than 8.3.7

Regression introduced with 8.3.11 commit:
drbd: Take a more conservative approach when deciding max_bio_size

Never ever tell an older drbd, that we support more than 32KiB
in a single data request (packet).
Never believe an older drbd, that is supports more than 32KiB
in a single data request (packet)
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_int.h

@@ -1284,7 +1284,8 @@ struct bm_extent {
 #endif
 #define DRBD_MAX_BIO_SIZE_SAFE (1 << 12)       /* Works always = 4k */
 
-#define DRBD_MAX_SIZE_H80_PACKET (1 << 15) /* The old header only allows packets up to 32Kib data */
+#define DRBD_MAX_SIZE_H80_PACKET (1 << 15) /* Header 80 only allows packets up to 32KiB data */
+#define DRBD_MAX_BIO_SIZE_P95    (1 << 17) /* Protocol 95 to 99 allows bios up to 128KiB */
 
 extern int  drbd_bm_init(struct drbd_conf *mdev);
 extern int  drbd_bm_resize(struct drbd_conf *mdev, sector_t sectors, int set_new_bits);

drivers/block/drbd/drbd_main.c

@@ -1075,6 +1075,12 @@ int drbd_send_sizes(struct drbd_conf *mdev, int trigger_reply, enum dds_flags fl
 	p = drbd_prepare_command(mdev, sock);
 	if (!p)
 		return -EIO;
+
+	if (mdev->tconn->agreed_pro_version <= 94)
+		max_bio_size = min_t(int, max_bio_size, DRBD_MAX_SIZE_H80_PACKET);
+	else if (mdev->tconn->agreed_pro_version < 100)
+		max_bio_size = min_t(int, max_bio_size, DRBD_MAX_BIO_SIZE_P95);
+
 	p->d_size = cpu_to_be64(d_size);
 	p->u_size = cpu_to_be64(u_size);
 	p->c_size = cpu_to_be64(trigger_reply ? 0 : drbd_get_capacity(mdev->this_bdev));

drivers/block/drbd/drbd_nl.c

@@ -1062,10 +1062,13 @@ void drbd_reconsider_max_bio_size(struct drbd_conf *mdev)
 	   BIOs for a single peer_request */
 	if (mdev->state.conn >= C_CONNECTED) {
 		if (mdev->tconn->agreed_pro_version < 94)
-			peer = mdev->peer_max_bio_size;
+			peer = min_t(int, mdev->peer_max_bio_size, DRBD_MAX_SIZE_H80_PACKET);
+			/* Correct old drbd (up to 8.3.7) if it believes it can do more than 32KiB */
 		else if (mdev->tconn->agreed_pro_version == 94)
 			peer = DRBD_MAX_SIZE_H80_PACKET;
-		else /* drbd 8.3.8 onwards */
+		else if (mdev->tconn->agreed_pro_version < 100)
+			peer = DRBD_MAX_BIO_SIZE_P95;  /* drbd 8.3.8 onwards, before 8.4.0 */
+		else
 			peer = DRBD_MAX_BIO_SIZE;
 	}