[PATCH] target: iscsi: use GFP_NOIO with loopback connections

[Maurizio Lombardi <mlombard@redhat.com>]

When an iscsi initiator is connected to a target running on the
same machine, the system may trigger a deadlock when working
under memory pressure.
This may happen, for example, when the iscsi rx thread tries to
allocate memory and a memory reclaim is performed, the rx thread may
therefore end up waiting for the initiator to complete I/O operations,
causing a deadlock.

Fix the issue by using memalloc_noio_*() to enable implicit GFP_NOIO
in the vulnerable code paths, when the connection is in loopback.

Suggested-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
---
 drivers/target/iscsi/iscsi_target.c       | 19 ++++++++++++++++---
 drivers/target/iscsi/iscsi_target_login.c |  8 ++++++++
 include/target/iscsi/iscsi_target_core.h  |  1 +
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index baf4da7bb3b4..4d997a049bf7 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -3918,9 +3918,9 @@ static int iscsit_handle_response_queue(struct iscsit_conn *conn)
 
 int iscsi_target_tx_thread(void *arg)
 {
-	int ret = 0;
+	int ret = 0, flags;
 	struct iscsit_conn *conn = arg;
-	bool conn_freed = false;
+	bool conn_freed = false, loopback;
 
 	/*
 	 * Allow ourselves to be interrupted by SIGINT so that a
@@ -3928,6 +3928,10 @@ int iscsi_target_tx_thread(void *arg)
 	 */
 	allow_signal(SIGINT);
 
+	loopback = conn->loopback;
+	if (loopback)
+		flags = memalloc_noio_save();
+
 	while (!kthread_should_stop()) {
 		/*
 		 * Ensure that both TX and RX per connection kthreads
@@ -3966,6 +3970,9 @@ int iscsi_target_tx_thread(void *arg)
 	if (conn->conn_state != TARG_CONN_STATE_IN_LOGIN)
 		iscsit_take_action_for_connection_exit(conn, &conn_freed);
 out:
+	if (loopback)
+		memalloc_noio_restore(flags);
+
 	if (!conn_freed) {
 		while (!kthread_should_stop()) {
 			msleep(100);
@@ -4166,7 +4173,7 @@ static void iscsit_get_rx_pdu(struct iscsit_conn *conn)
 
 int iscsi_target_rx_thread(void *arg)
 {
-	int rc;
+	int rc, flags;
 	struct iscsit_conn *conn = arg;
 	bool conn_freed = false;
 
@@ -4186,8 +4193,14 @@ int iscsi_target_rx_thread(void *arg)
 	if (!conn->conn_transport->iscsit_get_rx_pdu)
 		return 0;
 
+	if (conn->loopback)
+		flags = memalloc_noio_save();
+
 	conn->conn_transport->iscsit_get_rx_pdu(conn);
 
+	if (conn->loopback)
+		memalloc_noio_restore(flags);
+
 	if (!signal_pending(current))
 		atomic_set(&conn->transport_failed, 1);
 	iscsit_take_action_for_connection_exit(conn, &conn_freed);
diff --git a/drivers/target/iscsi/iscsi_target_login.c b/drivers/target/iscsi/iscsi_target_login.c
index 27e448c2d066..bbda125f6526 100644
--- a/drivers/target/iscsi/iscsi_target_login.c
+++ b/drivers/target/iscsi/iscsi_target_login.c
@@ -17,6 +17,7 @@
 #include <linux/tcp.h>        /* TCP_NODELAY */
 #include <net/ip.h>
 #include <net/ipv6.h>         /* ipv6_addr_v4mapped() */
+#include <net/sock.h>
 #include <scsi/iscsi_proto.h>
 #include <target/target_core_base.h>
 #include <target/target_core_fabric.h>
@@ -1246,6 +1247,7 @@ static int __iscsi_target_login_thread(struct iscsi_np *np)
 	struct iscsi_portal_group *tpg = NULL;
 	struct iscsi_login_req *pdu;
 	struct iscsi_tpg_np *tpg_np;
+	struct dst_entry *dst;
 	bool new_sess = false;
 
 	flush_signals(current);
@@ -1289,6 +1291,12 @@ static int __iscsi_target_login_thread(struct iscsi_np *np)
 		iscsit_free_conn(conn);
 		return 1;
 	}
+
+	dst = sk_dst_get(conn->sock->sk);
+	if (dst && dst->dev && dst->dev->flags & IFF_LOOPBACK)
+		conn->loopback = true;
+	dst_release(dst);
+
 	/*
 	 * Perform the remaining iSCSI connection initialization items..
 	 */
diff --git a/include/target/iscsi/iscsi_target_core.h b/include/target/iscsi/iscsi_target_core.h
index 94d06ddfd80a..aa8d4026e32e 100644
--- a/include/target/iscsi/iscsi_target_core.h
+++ b/include/target/iscsi/iscsi_target_core.h
@@ -538,6 +538,7 @@ struct iscsit_conn {
 	struct sockaddr_storage local_sockaddr;
 	int			conn_usage_count;
 	int			conn_waiting_on_uc;
+	bool			loopback;
 	atomic_t		check_immediate_queue;
 	atomic_t		conn_logout_remove;
 	atomic_t		connection_exit;
-- 
2.31.1