[PATCH v3 4/6] RDMA/cm: Brute force GFP_NOIO

["Håkon Bugge" <haakon.bugge@oracle.com>]

In ib_cm_init(), we call memalloc_noio_{save,restore} in a parenthetic
fashion when enabled by the module parameter force_noio.

This in order to conditionally enable ib_cm to work aligned with block
I/O devices. Any work queued later on work-queues created during
module initialization will inherit the PF_MEMALLOC_{NOIO,NOFS}
flag(s), due to commit ("workqueue: Inherit NOIO and NOFS alloc
flags").

We do this in order to enable ULPs using the RDMA stack to be used as
a network block I/O device. This to support a filesystem on top of a
raw block device which uses said ULP(s) and the RDMA stack as the
network transport layer.

Under intense memory pressure, we get memory reclaims. Assume the
filesystem reclaims memory, goes to the raw block device, which calls
into the ULP in question, which calls the RDMA stack. Now, if regular
GFP_KERNEL allocations in ULP or the RDMA stack require reclaims to be
fulfilled, we end up in a circular dependency.

We break this circular dependency by:

1. Force all allocations in the ULP and the relevant RDMA stack to use
   GFP_NOIO, by means of a parenthetic use of
   memalloc_noio_{save,restore} on all relevant entry points.

2. Make sure work-queues inherits current->flags
   wrt. PF_MEMALLOC_{NOIO,NOFS}, such that work executed on the
   work-queue inherits the same flag(s).

Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
---
 drivers/infiniband/core/cm.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/core/cm.c b/drivers/infiniband/core/cm.c
index 07fb8d3c037f0..767eec38eb57d 100644
--- a/drivers/infiniband/core/cm.c
+++ b/drivers/infiniband/core/cm.c
@@ -22,6 +22,7 @@
 #include <linux/workqueue.h>
 #include <linux/kdev_t.h>
 #include <linux/etherdevice.h>
+#include <linux/sched/mm.h>
 
 #include <rdma/ib_cache.h>
 #include <rdma/ib_cm.h>
@@ -35,6 +36,11 @@ MODULE_DESCRIPTION("InfiniBand CM");
 MODULE_LICENSE("Dual BSD/GPL");
 
 #define CM_DESTROY_ID_WAIT_TIMEOUT 10000 /* msecs */
+
+static bool cm_force_noio;
+module_param_named(force_noio, cm_force_noio, bool, 0444);
+MODULE_PARM_DESC(force_noio, "Force the use of GFP_NOIO (Y/N)");
+
 static const char * const ibcm_rej_reason_strs[] = {
 	[IB_CM_REJ_NO_QP]			= "no QP",
 	[IB_CM_REJ_NO_EEC]			= "no EEC",
@@ -4504,6 +4510,10 @@ static void cm_remove_one(struct ib_device *ib_device, void *client_data)
 static int __init ib_cm_init(void)
 {
 	int ret;
+	unsigned int noio_flags;
+
+	if (cm_force_noio)
+		noio_flags = memalloc_noio_save();
 
 	INIT_LIST_HEAD(&cm.device_list);
 	rwlock_init(&cm.device_lock);
@@ -4527,10 +4537,13 @@ static int __init ib_cm_init(void)
 	if (ret)
 		goto error3;
 
-	return 0;
+	goto error2;
 error3:
 	destroy_workqueue(cm.wq);
 error2:
+	if (cm_force_noio)
+		memalloc_noio_restore(noio_flags);
+
 	return ret;
 }
 
-- 
2.31.1