[PATCH] scsi: target: iscsi: set memalloc_noio with loopback network connections

[David Jeffery <djeffery@redhat.com>]

If an admin connects an iscsi initiator to an iscsi target on the same
system, the iscsi connection is vulnerable to deadlocks during memory
allocations. Memory allocations in the target task accepting the I/O from
the initiator can wait on the initiator's I/O when the system is under
memory pressure, causing a deadlock situation between the iscsi target and
initiator.

When in this configuration, the deadlock scenario can be avoided by use of
GFP_NOIO allocations. Rather than force all configurations to use NOIO,
memalloc_noio_save/restore can be used to force GFP_NOIO allocations only
when in this loopback configuration.

Signed-off-by: David Jeffery <djeffery@redhat.com>
---
 drivers/target/iscsi/iscsi_target.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index baf4da7bb3b4..a68e47e2cdf9 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -16,6 +16,7 @@
 #include <linux/vmalloc.h>
 #include <linux/idr.h>
 #include <linux/delay.h>
+#include <linux/sched/mm.h>
 #include <linux/sched/signal.h>
 #include <asm/unaligned.h>
 #include <linux/inet.h>
@@ -4168,7 +4169,10 @@ int iscsi_target_rx_thread(void *arg)
 {
 	int rc;
 	struct iscsit_conn *conn = arg;
+	struct dst_entry *dst;
 	bool conn_freed = false;
+	bool loopback = false;
+	unsigned int flags;
 
 	/*
 	 * Allow ourselves to be interrupted by SIGINT so that a
@@ -4186,8 +4190,25 @@ int iscsi_target_rx_thread(void *arg)
 	if (!conn->conn_transport->iscsit_get_rx_pdu)
 		return 0;
 
+	/*
+	 * If the iscsi connection is over a loopback device from using
+	 * iscsi and iscsit on the same system, we need to set memalloc_noio to
+	 * prevent memory allocation deadlocks between target and initiator.
+	 */
+	rcu_read_lock();
+	dst = rcu_dereference(conn->sock->sk->sk_dst_cache);
+	if (dst && dst->dev && dst->dev->flags & IFF_LOOPBACK)
+		loopback = true;
+	rcu_read_unlock();
+
+	if (loopback)
+		flags = memalloc_noio_save();
+
 	conn->conn_transport->iscsit_get_rx_pdu(conn);
 
+	if (loopback)
+		memalloc_noio_restore(flags);
+
 	if (!signal_pending(current))
 		atomic_set(&conn->transport_failed, 1);
 	iscsit_take_action_for_connection_exit(conn, &conn_freed);
-- 
2.39.1