[PATCH v2 6/6] net/mlx5: Brute force GFP_NOIO

[Håkon Bugge <haakon.bugge@xxxxxxxxxx>]

In mlx5_core_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 mlx5_core to work aligned with
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 and the
mlx5_core driver 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@xxxxxxxxxx>
---
 drivers/net/ethernet/mellanox/mlx5/core/main.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/main.c b/drivers/net/ethernet/mellanox/mlx5/core/main.c
index 331ce47f51a17..aa1bf8bb5d15c 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/main.c
@@ -48,6 +48,7 @@
 #include <linux/mlx5/vport.h>
 #include <linux/version.h>
 #include <net/devlink.h>
+#include <linux/sched/mm.h>
 #include "mlx5_core.h"
 #include "lib/eq.h"
 #include "fs_core.h"
@@ -87,6 +88,10 @@ static unsigned int prof_sel = MLX5_DEFAULT_PROF;
 module_param_named(prof_sel, prof_sel, uint, 0444);
 MODULE_PARM_DESC(prof_sel, "profile selector. Valid range 0 - 2");
 
+static bool mlx5_core_force_noio;
+module_param_named(force_noio, mlx5_core_force_noio, bool, 0444);
+MODULE_PARM_DESC(force_noio, "Force the use of GFP_NOIO (Y/N)");
+
 static u32 sw_owner_id[4];
 #define MAX_SW_VHCA_ID (BIT(__mlx5_bit_sz(cmd_hca_cap_2, sw_vhca_id)) - 1)
 static DEFINE_IDA(sw_vhca_ida);
@@ -2312,8 +2317,12 @@ static void mlx5_core_verify_params(void)
 
 static int __init mlx5_init(void)
 {
+	unsigned int noio_flags;
 	int err;
 
+	if (mlx5_core_force_noio)
+		noio_flags = memalloc_noio_save();
+
 	WARN_ONCE(strcmp(MLX5_ADEV_NAME, KBUILD_MODNAME),
 		  "mlx5_core name not in sync with kernel module name");
 
@@ -2334,7 +2343,7 @@ static int __init mlx5_init(void)
 	if (err)
 		goto err_pci;
 
-	return 0;
+	goto out;
 
 err_pci:
 	mlx5_sf_driver_unregister();
@@ -2342,6 +2351,9 @@ static int __init mlx5_init(void)
 	mlx5e_cleanup();
 err_debug:
 	mlx5_unregister_debugfs();
+out:
+	if (mlx5_core_force_noio)
+		memalloc_noio_restore(noio_flags);
 	return err;
 }
 
-- 
2.45.0