drbd: report congestion if we are waiting for some userland callback

If the drbd worker thread is synchronously waiting for some userland
callback, we don't want some casual pageout to block on us.
Have drbd_congested() report congestion in that case.

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

3 files changed, 30 insertions, 1 deletions

diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index 963766bafab4..e010afff336a 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -815,6 +815,12 @@ enum {
 	CONN_DRY_RUN,		/* Expect disconnect after resync handshake. */
 	CREATE_BARRIER,		/* next P_DATA is preceded by a P_BARRIER */
 	STATE_SENT,		/* Do not change state/UUIDs while this is set */
+	CALLBACK_PENDING,	/* Whether we have a call_usermodehelper(, UMH_WAIT_PROC)
+				 * pending, from drbd worker context.
+				 * If set, bdi_write_congested() returns true,
+				 * so shrink_page_list() would not recurse into,
+				 * and potentially deadlock on, this drbd worker.
+				 */
 };
 
 struct drbd_tconn {			/* is a resource from the config file */
diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index 849e5de9ea8f..f2af74d06860 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -2338,6 +2338,22 @@ static int drbd_congested(void *congested_data, int bdi_bits)
 		goto out;
 	}
 
+	if (test_bit(CALLBACK_PENDING, &mdev->tconn->flags)) {
+		r |= (1 << BDI_async_congested);
+		/* Without good local data, we would need to read from remote,
+		 * and that would need the worker thread as well, which is
+		 * currently blocked waiting for that usermode helper to
+		 * finish.
+		 */
+		if (!get_ldev_if_state(mdev, D_UP_TO_DATE))
+			r |= (1 << BDI_sync_congested);
+		else
+			put_ldev(mdev);
+		r &= bdi_bits;
+		reason = 'c';
+		goto out;
+	}
+
 	if (get_ldev(mdev)) {
 		q = bdev_get_queue(mdev->ldev->backing_bdev);
 		r = bdi_congested(&q->backing_dev_info, bdi_bits);
diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index d4c05e26a13a..05ed4804c72c 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -323,11 +323,15 @@ int drbd_khelper(struct drbd_conf *mdev, char *cmd)
 			NULL };
 	char mb[12];
 	char *argv[] = {usermode_helper, cmd, mb, NULL };
+	struct drbd_tconn *tconn = mdev->tconn;
 	struct sib_info sib;
 	int ret;
 
+	if (current == tconn->worker.task)
+		set_bit(CALLBACK_PENDING, &tconn->flags);
+
 	snprintf(mb, 12, "minor-%d", mdev_to_minor(mdev));
-	setup_khelper_env(mdev->tconn, envp);
+	setup_khelper_env(tconn, envp);
 
 	/* The helper may take some time.
 	 * write out any unsynced meta data changes now */
@@ -350,6 +354,9 @@ int drbd_khelper(struct drbd_conf *mdev, char *cmd)
 	sib.helper_exit_code = ret;
 	drbd_bcast_event(mdev, &sib);
 
+	if (current == tconn->worker.task)
+		clear_bit(CALLBACK_PENDING, &tconn->flags);
+
 	if (ret < 0) /* Ignore any ERRNOs we got. */
 		ret = 0;