[PATCH v2 3/6] RDMA/cma: Brute force GFP_NOIO

[Håkon Bugge <haakon.bugge@xxxxxxxxxx>]

In cma_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 rdma_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 the 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@xxxxxxxxxx>
---
 drivers/infiniband/core/cma.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/infiniband/core/cma.c b/drivers/infiniband/core/cma.c
index 1e2cd7c8716e8..23a50cc3e81cb 100644
--- a/drivers/infiniband/core/cma.c
+++ b/drivers/infiniband/core/cma.c
@@ -50,6 +50,10 @@ MODULE_LICENSE("Dual BSD/GPL");
 #define CMA_IBOE_PACKET_LIFETIME 16
 #define CMA_PREFERRED_ROCE_GID_TYPE IB_GID_TYPE_ROCE_UDP_ENCAP
 
+static bool cma_force_noio;
+module_param_named(force_noio, cma_force_noio, bool, 0444);
+MODULE_PARM_DESC(force_noio, "Force the use of GFP_NOIO (Y/N)");
+
 static const char * const cma_events[] = {
 	[RDMA_CM_EVENT_ADDR_RESOLVED]	 = "address resolved",
 	[RDMA_CM_EVENT_ADDR_ERROR]	 = "address error",
@@ -5424,6 +5428,10 @@ static struct pernet_operations cma_pernet_operations = {
 static int __init cma_init(void)
 {
 	int ret;
+	unsigned int noio_flags;
+
+	if (cma_force_noio)
+		noio_flags = memalloc_noio_save();
 
 	/*
 	 * There is a rare lock ordering dependency in cma_netdev_callback()
@@ -5439,8 +5447,10 @@ static int __init cma_init(void)
 	}
 
 	cma_wq = alloc_ordered_workqueue("rdma_cm", WQ_MEM_RECLAIM);
-	if (!cma_wq)
-		return -ENOMEM;
+	if (!cma_wq) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	ret = register_pernet_subsys(&cma_pernet_operations);
 	if (ret)
@@ -5458,7 +5468,8 @@ static int __init cma_init(void)
 	if (ret)
 		goto err_ib;
 
-	return 0;
+	ret = 0;
+	goto out;
 
 err_ib:
 	ib_unregister_client(&cma_client);
@@ -5469,6 +5480,9 @@ static int __init cma_init(void)
 	unregister_pernet_subsys(&cma_pernet_operations);
 err_wq:
 	destroy_workqueue(cma_wq);
+out:
+	if (cma_force_noio)
+		memalloc_noio_restore(noio_flags);
 	return ret;
 }
 
-- 
2.45.0