[PATCH net-next 0/4] net: devmem: allow rx-buf-size > PAGE_SIZE per binding

[PATCH net-next 0/4] net: devmem: allow rx-buf-size > PAGE_SIZE per binding

[Bobby Eshleman]

Every devmem dmabuf binding hands the page_pool PAGE_SIZE niovs today.
On NICs that consume one descriptor per netmem, this caps a single RX
descriptor at PAGE_SIZE and burns CPU on buffer churn.

In this series, we add a bind-time netlink attribute,
NETDEV_A_DMABUF_RX_BUF_SIZE, that lets userspace request a larger niov size
(power of two >= PAGE_SIZE). Drivers must opt in via
queue_mgmt_ops.QCFG_RX_PAGE_SIZE.

Selftests use udmabuf, but udmabuf sgtables were previously hardcoded to
PAGE_SIZE. This series modifies udmabuf to respect folio sizes in its exported
sgtable. The result is that when backing udmabuf with MFD_HUGETLB 2MB pages,
the sgtable is populated with 2MB entries, allowing devmem's gen_pool to carve
out large (eg. 64K) niovs.

Measurements
------------

Setup: kperf devmem RX/TX cuda, 4 flows, 64 MB messages, 60s, dctcp,
num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov size,
mlx5.

   niov       RX dev Gbps   RX flow avg Gbps         app sys %
  -----  ----------------  -----------------  ----------------
     4K  300.63 +/- 53.21    75.16 +/- 13.30   54.15 +/- 10.23
    16K  321.35 +/- 28.20    80.34 +/-  7.05   41.05 +/-  8.87
    32K  347.63 +/-  2.20    86.91 +/-  0.55   44.54 +/-  3.51
    64K  332.11 +/- 14.26    83.03 +/-  3.56   35.47 +/-  3.11

RX app sys % drops ~19% from 4K to 64K.

kperf support (not yet merged):
https://github.com/facebookexperimental/kperf/commit/8837577f920876bce6986ec18869ac04439ebcd2

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
Bobby Eshleman (4):
      net: devmem: allow rx-buf-size > PAGE_SIZE per dmabuf binding
      udmabuf: emit one sg entry per pinned folio
      selftests/net: ncdevmem: add -b option to set rx-buf-size on bind
      selftests/net: devmem.py: add check_rx_large_niov

 Documentation/netlink/specs/netdev.yaml            |  8 ++++
 drivers/dma-buf/udmabuf.c                          | 47 ++++++++++++++++---
 include/uapi/linux/netdev.h                        |  1 +
 net/core/devmem.c                                  | 52 +++++++++++++---------
 net/core/devmem.h                                  | 13 ++++--
 net/core/netdev-genl-gen.c                         |  5 ++-
 net/core/netdev-genl.c                             | 18 +++++++-
 tools/include/uapi/linux/netdev.h                  |  1 +
 tools/testing/selftests/drivers/net/hw/config      |  1 +
 tools/testing/selftests/drivers/net/hw/devmem.py   | 12 ++++-
 .../testing/selftests/drivers/net/hw/devmem_lib.py | 46 ++++++++++++++++++-
 tools/testing/selftests/drivers/net/hw/ncdevmem.c  | 49 ++++++++++++++++++--
 .../testing/selftests/drivers/net/hw/nk_devmem.py  | 11 ++++-
 13 files changed, 220 insertions(+), 44 deletions(-)
---
base-commit: dfcc2ff12925d99e858eaf539eaa4aaaf81fe2a6
change-id: 20260602-tcpdm-large-niovs-56523a3a1077

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@meta.com>

[PATCH net-next 1/4] net: devmem: allow rx-buf-size > PAGE_SIZE per dmabuf binding

[Bobby Eshleman]

From: Bobby Eshleman <bobbyeshleman@meta.com>

Every devmem dmabuf binding today hands the page_pool PAGE_SIZE niovs.
This caps a single RX descriptor at PAGE_SIZE, burning CPU on buffer
churn for large flows.

Add a bind-time netlink attribute, NETDEV_A_DMABUF_RX_BUF_SIZE, that
lets userspace request a larger niov size. The value must be a power of
two >= PAGE_SIZE.

Measurements
------------
Setup: kperf in devmem RX/TX cuda mode, 4 flows, 64 MB messages, 60s,
dctcp, num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov
size, mlx5.

CPU Util:

   niov        net sirq %        net idle %         app sys %        app idle %
  -----  ----------------  ----------------  ----------------  ----------------
     4K   62.38 +/-  8.27   33.40 +/-  7.51   54.15 +/- 10.23   43.67 +/- 10.53
    16K   58.91 +/-  5.35   35.23 +/-  5.88   41.05 +/-  8.87   56.42 +/-  9.24
    32K   64.12 +/-  0.68   31.09 +/-  1.48   44.54 +/-  3.51   52.63 +/-  3.65
    64K   54.69 +/-  5.54   39.67 +/-  5.81   35.47 +/-  3.11   61.97 +/-  3.27

RX app sys % drops ~19% from 4K to 64K.

Throughput:

   niov       RX dev Gbps   RX flow avg Gbps
  -----  ----------------  -----------------
     4K  300.63 +/- 53.21    75.16 +/- 13.30
    16K  321.35 +/- 28.20    80.34 +/-  7.05
    32K  347.63 +/-  2.20    86.91 +/-  0.55
    64K  332.11 +/- 14.26    83.03 +/-  3.56

Throughput seems to increase, but the stdev is pretty wide so could just
be noise.

kperf support (not yet merged):
https://github.com/facebookexperimental/kperf/commit/8837577f920876bce6986ec18869ac04439ebcd2

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 Documentation/netlink/specs/netdev.yaml |  8 +++++
 include/uapi/linux/netdev.h             |  1 +
 net/core/devmem.c                       | 52 +++++++++++++++++++--------------
 net/core/devmem.h                       | 13 ++++++---
 net/core/netdev-genl-gen.c              |  5 ++--
 net/core/netdev-genl.c                  | 18 ++++++++++--
 tools/include/uapi/linux/netdev.h       |  1 +
 7 files changed, 68 insertions(+), 30 deletions(-)

diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
index a1f4c5a561e9..063119907983 100644
--- a/Documentation/netlink/specs/netdev.yaml
+++ b/Documentation/netlink/specs/netdev.yaml
@@ -591,6 +591,13 @@ attribute-sets:
         type: u32
         checks:
           min: 1
+      -
+        name: rx-buf-size
+        doc: |
+          Size in bytes of each RX buffer the NIC writes into from the bound
+          dmabuf. Must be a power of two and >= PAGE_SIZE; defaults to
+          PAGE_SIZE.
+        type: u32
 
 operations:
   list:
@@ -805,6 +812,7 @@ operations:
             - ifindex
             - fd
             - queues
+            - rx-buf-size
         reply:
           attributes:
             - id
diff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h
index 7df1056a35fd..180a4ffffd60 100644
--- a/include/uapi/linux/netdev.h
+++ b/include/uapi/linux/netdev.h
@@ -217,6 +217,7 @@ enum {
 	NETDEV_A_DMABUF_QUEUES,
 	NETDEV_A_DMABUF_FD,
 	NETDEV_A_DMABUF_ID,
+	NETDEV_A_DMABUF_RX_BUF_SIZE,
 
 	__NETDEV_A_DMABUF_MAX,
 	NETDEV_A_DMABUF_MAX = (__NETDEV_A_DMABUF_MAX - 1)
diff --git a/net/core/devmem.c b/net/core/devmem.c
index 957d6b96216b..5a1c0d7984a8 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -46,7 +46,7 @@ static dma_addr_t net_devmem_get_dma_addr(const struct net_iov *niov)
 
 	owner = net_devmem_iov_to_chunk_owner(niov);
 	return owner->base_dma_addr +
-	       ((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT);
+	       ((dma_addr_t)net_iov_idx(niov) << owner->binding->niov_shift);
 }
 
 static void net_devmem_dmabuf_binding_release(struct percpu_ref *ref)
@@ -93,13 +93,14 @@ net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding)
 	ssize_t offset;
 	ssize_t index;
 
-	dma_addr = gen_pool_alloc_owner(binding->chunk_pool, PAGE_SIZE,
+	dma_addr = gen_pool_alloc_owner(binding->chunk_pool,
+					1UL << binding->niov_shift,
 					(void **)&owner);
 	if (!dma_addr)
 		return NULL;
 
 	offset = dma_addr - owner->base_dma_addr;
-	index = offset / PAGE_SIZE;
+	index = offset >> binding->niov_shift;
 	niov = &owner->area.niovs[index];
 
 	niov->desc.pp_magic = 0;
@@ -113,12 +114,13 @@ void net_devmem_free_dmabuf(struct net_iov *niov)
 {
 	struct net_devmem_dmabuf_binding *binding = net_devmem_iov_binding(niov);
 	unsigned long dma_addr = net_devmem_get_dma_addr(niov);
+	size_t niov_size = 1UL << binding->niov_shift;
 
 	if (WARN_ON(!gen_pool_has_addr(binding->chunk_pool, dma_addr,
-				       PAGE_SIZE)))
+				       niov_size)))
 		return;
 
-	gen_pool_free(binding->chunk_pool, dma_addr, PAGE_SIZE);
+	gen_pool_free(binding->chunk_pool, dma_addr, niov_size);
 }
 
 void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding)
@@ -163,6 +165,9 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
 	u32 xa_idx;
 	int err;
 
+	if (binding->niov_shift != PAGE_SHIFT)
+		mp_params.rx_page_size = 1U << binding->niov_shift;
+
 	err = netif_mp_open_rxq(dev, rxq_idx, &mp_params, extack);
 	if (err)
 		return err;
@@ -184,14 +189,16 @@ struct net_devmem_dmabuf_binding *
 net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 		       struct device *dma_dev,
 		       enum dma_data_direction direction,
-		       unsigned int dmabuf_fd, struct netdev_nl_sock *priv,
+		       unsigned int dmabuf_fd, unsigned int niov_shift,
+		       struct netdev_nl_sock *priv,
 		       struct netlink_ext_ack *extack)
 {
 	struct net_devmem_dmabuf_binding *binding;
+	size_t niov_size = 1UL << niov_shift;
 	static u32 id_alloc_next;
+	unsigned int sg_idx, i;
 	struct scatterlist *sg;
 	struct dma_buf *dmabuf;
-	unsigned int sg_idx, i;
 	unsigned long virtual;
 	int err;
 
@@ -213,6 +220,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 
 	binding->dev = dev;
 	binding->vdev = vdev;
+	binding->niov_shift = niov_shift;
 	xa_init_flags(&binding->bound_rxqs, XA_FLAGS_ALLOC);
 
 	err = percpu_ref_init(&binding->ref,
@@ -248,18 +256,14 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 			goto err_unmap;
 		}
 		binding->tx_vec = kvmalloc_objs(struct net_iov *,
-						dmabuf->size / PAGE_SIZE);
+						dmabuf->size >> niov_shift);
 		if (!binding->tx_vec) {
 			err = -ENOMEM;
 			goto err_unmap;
 		}
 	}
 
-	/* For simplicity we expect to make PAGE_SIZE allocations, but the
-	 * binding can be much more flexible than that. We may be able to
-	 * allocate MTU sized chunks here. Leave that for future work...
-	 */
-	binding->chunk_pool = gen_pool_create(PAGE_SHIFT,
+	binding->chunk_pool = gen_pool_create(niov_shift,
 					      dev_to_node(&dev->dev));
 	if (!binding->chunk_pool) {
 		err = -ENOMEM;
@@ -273,9 +277,11 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 		size_t len = sg_dma_len(sg);
 		struct net_iov *niov;
 
-		if (!IS_ALIGNED(len, PAGE_SIZE)) {
+		if (!IS_ALIGNED(dma_addr, niov_size) ||
+		    !IS_ALIGNED(len, niov_size)) {
 			err = -EINVAL;
-			NL_SET_ERR_MSG(extack, "dma-buf SG length must be PAGE_SIZE aligned");
+			NL_SET_ERR_MSG(extack,
+				       "dmabuf sg entry not aligned to niov size");
 			goto err_free_chunks;
 		}
 
@@ -288,7 +294,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 
 		owner->area.base_virtual = virtual;
 		owner->base_dma_addr = dma_addr;
-		owner->area.num_niovs = len / PAGE_SIZE;
+		owner->area.num_niovs = len >> niov_shift;
 		owner->binding = binding;
 
 		err = gen_pool_add_owner(binding->chunk_pool, dma_addr,
@@ -313,7 +319,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 			page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov),
 						      net_devmem_get_dma_addr(niov));
 			if (direction == DMA_TO_DEVICE)
-				binding->tx_vec[owner->area.base_virtual / PAGE_SIZE + i] = niov;
+				binding->tx_vec[(owner->area.base_virtual >> niov_shift) + i] = niov;
 		}
 
 		virtual += len;
@@ -430,13 +436,15 @@ struct net_iov *
 net_devmem_get_niov_at(struct net_devmem_dmabuf_binding *binding,
 		       size_t virt_addr, size_t *off, size_t *size)
 {
+	size_t niov_size = 1UL << binding->niov_shift;
+
 	if (virt_addr >= binding->dmabuf->size)
 		return NULL;
 
-	*off = virt_addr % PAGE_SIZE;
-	*size = PAGE_SIZE - *off;
+	*off = virt_addr & (niov_size - 1);
+	*size = niov_size - *off;
 
-	return binding->tx_vec[virt_addr / PAGE_SIZE];
+	return binding->tx_vec[virt_addr >> binding->niov_shift];
 }
 
 /*** "Dmabuf devmem memory provider" ***/
@@ -454,8 +462,8 @@ int mp_dmabuf_devmem_init(struct page_pool *pool)
 	pool->dma_sync = false;
 	pool->dma_sync_for_cpu = false;
 
-	if (pool->p.order != 0)
-		return -E2BIG;
+	if (pool->p.order != binding->niov_shift - PAGE_SHIFT)
+		return -EINVAL;
 
 	net_devmem_dmabuf_binding_get(binding);
 	return 0;
diff --git a/net/core/devmem.h b/net/core/devmem.h
index 3852a56036cb..4a293a7d1149 100644
--- a/net/core/devmem.h
+++ b/net/core/devmem.h
@@ -71,6 +71,8 @@ struct net_devmem_dmabuf_binding {
 	 */
 	struct net_iov **tx_vec;
 
+	unsigned int niov_shift;
+
 	struct work_struct unbind_w;
 };
 
@@ -93,7 +95,8 @@ struct net_devmem_dmabuf_binding *
 net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 		       struct device *dma_dev,
 		       enum dma_data_direction direction,
-		       unsigned int dmabuf_fd, struct netdev_nl_sock *priv,
+		       unsigned int dmabuf_fd, unsigned int niov_shift,
+		       struct netdev_nl_sock *priv,
 		       struct netlink_ext_ack *extack);
 struct net_devmem_dmabuf_binding *net_devmem_lookup_dmabuf(u32 id);
 void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding);
@@ -122,10 +125,11 @@ static inline u32 net_devmem_iov_binding_id(const struct net_iov *niov)
 
 static inline unsigned long net_iov_virtual_addr(const struct net_iov *niov)
 {
-	struct net_iov_area *owner = net_iov_owner(niov);
+	struct dmabuf_genpool_chunk_owner *co =
+		net_devmem_iov_to_chunk_owner(niov);
 
-	return owner->base_virtual +
-	       ((unsigned long)net_iov_idx(niov) << PAGE_SHIFT);
+	return net_iov_owner(niov)->base_virtual +
+	       ((unsigned long)net_iov_idx(niov) << co->binding->niov_shift);
 }
 
 static inline bool
@@ -175,6 +179,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
 		       struct device *dma_dev,
 		       enum dma_data_direction direction,
 		       unsigned int dmabuf_fd,
+		       unsigned int niov_shift,
 		       struct netdev_nl_sock *priv,
 		       struct netlink_ext_ack *extack)
 {
diff --git a/net/core/netdev-genl-gen.c b/net/core/netdev-genl-gen.c
index c7e138bfe345..55e03b9cd227 100644
--- a/net/core/netdev-genl-gen.c
+++ b/net/core/netdev-genl-gen.c
@@ -106,10 +106,11 @@ static const struct nla_policy netdev_qstats_get_nl_policy[NETDEV_A_QSTATS_SCOPE
 };
 
 /* NETDEV_CMD_BIND_RX - do */
-static const struct nla_policy netdev_bind_rx_nl_policy[NETDEV_A_DMABUF_FD + 1] = {
+static const struct nla_policy netdev_bind_rx_nl_policy[NETDEV_A_DMABUF_RX_BUF_SIZE + 1] = {
 	[NETDEV_A_DMABUF_IFINDEX] = NLA_POLICY_MIN(NLA_U32, 1),
 	[NETDEV_A_DMABUF_FD] = { .type = NLA_U32, },
 	[NETDEV_A_DMABUF_QUEUES] = NLA_POLICY_NESTED(netdev_queue_id_nl_policy),
+	[NETDEV_A_DMABUF_RX_BUF_SIZE] = { .type = NLA_U32, },
 };
 
 /* NETDEV_CMD_NAPI_SET - do */
@@ -219,7 +220,7 @@ static const struct genl_split_ops netdev_nl_ops[] = {
 		.cmd		= NETDEV_CMD_BIND_RX,
 		.doit		= netdev_nl_bind_rx_doit,
 		.policy		= netdev_bind_rx_nl_policy,
-		.maxattr	= NETDEV_A_DMABUF_FD,
+		.maxattr	= NETDEV_A_DMABUF_RX_BUF_SIZE,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
 	},
 	{
diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
index b4d48f3672a5..9902a97698f5 100644
--- a/net/core/netdev-genl.c
+++ b/net/core/netdev-genl.c
@@ -1012,6 +1012,7 @@ int netdev_nl_bind_rx_doit(struct sk_buff *skb, struct genl_info *info)
 {
 	struct net_devmem_dmabuf_binding *binding;
 	u32 ifindex, dmabuf_fd, rxq_idx;
+	unsigned int niov_shift = PAGE_SHIFT;
 	struct netdev_nl_sock *priv;
 	struct net_device *netdev;
 	unsigned long *rxq_bitmap;
@@ -1028,6 +1029,18 @@ int netdev_nl_bind_rx_doit(struct sk_buff *skb, struct genl_info *info)
 	ifindex = nla_get_u32(info->attrs[NETDEV_A_DEV_IFINDEX]);
 	dmabuf_fd = nla_get_u32(info->attrs[NETDEV_A_DMABUF_FD]);
 
+	if (info->attrs[NETDEV_A_DMABUF_RX_BUF_SIZE]) {
+		u32 rx_buf_size = nla_get_u32(info->attrs[NETDEV_A_DMABUF_RX_BUF_SIZE]);
+
+		if (!rx_buf_size || !is_power_of_2(rx_buf_size) ||
+		    rx_buf_size < PAGE_SIZE) {
+			NL_SET_ERR_MSG(info->extack,
+				       "rx_buf_size must be a power of 2 >= PAGE_SIZE");
+			return -EINVAL;
+		}
+		niov_shift = ilog2(rx_buf_size);
+	}
+
 	priv = genl_sk_priv_get(&netdev_nl_family, NETLINK_CB(skb).sk);
 	if (IS_ERR(priv))
 		return PTR_ERR(priv);
@@ -1078,7 +1091,8 @@ int netdev_nl_bind_rx_doit(struct sk_buff *skb, struct genl_info *info)
 	}
 
 	binding = net_devmem_bind_dmabuf(netdev, NULL, dma_dev, DMA_FROM_DEVICE,
-					 dmabuf_fd, priv, info->extack);
+					 dmabuf_fd, niov_shift, priv,
+					 info->extack);
 	if (IS_ERR(binding)) {
 		err = PTR_ERR(binding);
 		goto err_rxq_bitmap;
@@ -1221,7 +1235,7 @@ int netdev_nl_bind_tx_doit(struct sk_buff *skb, struct genl_info *info)
 	binding = net_devmem_bind_dmabuf(bind_dev,
 					 bind_dev != netdev ? netdev : NULL,
 					 dma_dev, DMA_TO_DEVICE, dmabuf_fd,
-					 priv, info->extack);
+					 PAGE_SHIFT, priv, info->extack);
 	if (IS_ERR(binding)) {
 		err = PTR_ERR(binding);
 		goto err_unlock_bind_dev;
diff --git a/tools/include/uapi/linux/netdev.h b/tools/include/uapi/linux/netdev.h
index 7df1056a35fd..180a4ffffd60 100644
--- a/tools/include/uapi/linux/netdev.h
+++ b/tools/include/uapi/linux/netdev.h
@@ -217,6 +217,7 @@ enum {
 	NETDEV_A_DMABUF_QUEUES,
 	NETDEV_A_DMABUF_FD,
 	NETDEV_A_DMABUF_ID,
+	NETDEV_A_DMABUF_RX_BUF_SIZE,
 
 	__NETDEV_A_DMABUF_MAX,
 	NETDEV_A_DMABUF_MAX = (__NETDEV_A_DMABUF_MAX - 1)

-- 
2.53.0-Meta

[PATCH net-next 2/4] udmabuf: emit one sg entry per pinned folio

[Bobby Eshleman]

From: Bobby Eshleman <bobbyeshleman@meta.com>

get_sg_table() emitted one PAGE_SIZE sg entry per page even when the
underlying folio was larger.

Instead, walk folios[] and emit one sg entry per folio. When folios
represent large pages (as is for MFD_HUGETLB), each sg entry is a large
page. Normal PAGE_SIZE sg tables are unchanged.

Required by net/core/devmem to support rx-buf-size > PAGE_SIZE with
udmabuf.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 drivers/dma-buf/udmabuf.c | 47 ++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 42 insertions(+), 5 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index 94b8ecb892bb..f28dd3788ada 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -141,26 +141,63 @@ static void vunmap_udmabuf(struct dma_buf *buf, struct iosys_map *map)
 	vm_unmap_ram(map->vaddr, ubuf->pagecount);
 }
 
+/* Return the number of contiguous pages backed by the folio at @i.
+ * A udmabuf may map only part of a folio, or reference the same folio
+ * in multiple non-contiguous runs, so folio_nr_pages() can't be used.
+ */
+static pgoff_t udmabuf_folio_nr_pages(struct udmabuf *ubuf, pgoff_t i)
+{
+	struct folio *f = ubuf->folios[i];
+	pgoff_t j;
+
+	for (j = 1; i + j < ubuf->pagecount; j++) {
+		if (ubuf->folios[i + j] != f)
+			break;
+		/* Same folio, but not a sequential offset within it. */
+		if (ubuf->offsets[i + j] != ubuf->offsets[i] + j * PAGE_SIZE)
+			break;
+	}
+	return j;
+}
+
+/* Count the contiguous folio runs in @ubuf, one sg entry per run. */
+static unsigned int udmabuf_sg_nents(struct udmabuf *ubuf)
+{
+	unsigned int nents = 0;
+	pgoff_t i;
+
+	for (i = 0; i < ubuf->pagecount; i += udmabuf_folio_nr_pages(ubuf, i))
+		nents++;
+	return nents;
+}
+
 static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
 				     enum dma_data_direction direction)
 {
 	struct udmabuf *ubuf = buf->priv;
-	struct sg_table *sg;
 	struct scatterlist *sgl;
-	unsigned int i = 0;
+	struct sg_table *sg;
+	pgoff_t i, run;
+	unsigned int nents;
 	int ret;
 
+	nents = udmabuf_sg_nents(ubuf);
+
 	sg = kzalloc_obj(*sg);
 	if (!sg)
 		return ERR_PTR(-ENOMEM);
 
-	ret = sg_alloc_table(sg, ubuf->pagecount, GFP_KERNEL);
+	ret = sg_alloc_table(sg, nents, GFP_KERNEL);
 	if (ret < 0)
 		goto err_alloc;
 
-	for_each_sg(sg->sgl, sgl, ubuf->pagecount, i)
-		sg_set_folio(sgl, ubuf->folios[i], PAGE_SIZE,
+	sgl = sg->sgl;
+	for (i = 0; i < ubuf->pagecount; i += run) {
+		run = udmabuf_folio_nr_pages(ubuf, i);
+		sg_set_folio(sgl, ubuf->folios[i], run << PAGE_SHIFT,
 			     ubuf->offsets[i]);
+		sgl = sg_next(sgl);
+	}
 
 	ret = dma_map_sgtable(dev, sg, direction, 0);
 	if (ret < 0)

-- 
2.53.0-Meta

[PATCH net-next 3/4] selftests/net: ncdevmem: add -b option to set rx-buf-size on bind

[Bobby Eshleman]

From: Bobby Eshleman <bobbyeshleman@meta.com>

Add -b <bytes> to request a non-default niov size via
NETDEV_A_DMABUF_RX_BUF_SIZE. When the value exceeds PAGE_SIZE,
udmabuf_alloc() switches to an MFD_HUGETLB-backed memfd so each 2 MB
hugepage produces one naturally-aligned sg entry.

Reject values > 2 MB up front: MFD_HUGETLB + udmabuf can only guarantee
2 MB per sg entry (one hugepage), so a larger rx_buf_size would fail the
per-sg length/alignment check.

Add CONFIG_HUGETLBFS=y to drivers/net/hw/config so the new path is
reachable in the CI kernels built for these tests.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 tools/testing/selftests/drivers/net/hw/config     |  1 +
 tools/testing/selftests/drivers/net/hw/ncdevmem.c | 49 +++++++++++++++++++++--
 2 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/drivers/net/hw/config b/tools/testing/selftests/drivers/net/hw/config
index b9f406dd7282..388721bee553 100644
--- a/tools/testing/selftests/drivers/net/hw/config
+++ b/tools/testing/selftests/drivers/net/hw/config
@@ -3,6 +3,7 @@ CONFIG_FAIL_FUNCTION=y
 CONFIG_FAULT_INJECTION=y
 CONFIG_FAULT_INJECTION_DEBUG_FS=y
 CONFIG_FUNCTION_ERROR_INJECTION=y
+CONFIG_HUGETLBFS=y
 CONFIG_INET6_ESP=y
 CONFIG_INET6_ESP_OFFLOAD=y
 CONFIG_INET_ESP=y
diff --git a/tools/testing/selftests/drivers/net/hw/ncdevmem.c b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
index d96e8a3b5a65..325c128191e2 100644
--- a/tools/testing/selftests/drivers/net/hw/ncdevmem.c
+++ b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
@@ -61,6 +61,7 @@
 #include <sys/time.h>
 
 #include <linux/memfd.h>
+#include <sys/param.h>
 #include <linux/dma-buf.h>
 #include <linux/errqueue.h>
 #include <linux/udmabuf.h>
@@ -79,6 +80,7 @@
 #define PAGE_SHIFT 12
 #define TEST_PREFIX "ncdevmem"
 #define NUM_PAGES 16000
+#define MB(x) ((x) << 20)
 
 #ifndef MSG_SOCK_DEVMEM
 #define MSG_SOCK_DEVMEM 0x2000000
@@ -100,6 +102,7 @@ static unsigned int dmabuf_id;
 static uint32_t tx_dmabuf_id;
 static int waittime_ms = 500;
 static bool fail_on_linear;
+static uint32_t rx_buf_size;
 
 /* System state loaded by current_config_load() */
 #define MAX_FLOWS	8
@@ -142,6 +145,7 @@ static struct memory_buffer *udmabuf_alloc(size_t size)
 {
 	struct udmabuf_create create;
 	struct memory_buffer *ctx;
+	unsigned int memfd_flags;
 	int ret;
 
 	ctx = malloc(sizeof(*ctx));
@@ -156,9 +160,14 @@ static struct memory_buffer *udmabuf_alloc(size_t size)
 		goto err_free_ctx;
 	}
 
-	ctx->memfd = memfd_create("udmabuf-test", MFD_ALLOW_SEALING);
+	memfd_flags = MFD_ALLOW_SEALING;
+	if (rx_buf_size > (uint32_t)getpagesize())
+		memfd_flags |= MFD_HUGETLB | MFD_HUGE_2MB;
+
+	ctx->memfd = memfd_create("udmabuf-test", memfd_flags);
 	if (ctx->memfd < 0) {
-		pr_err("[skip,no-memfd]");
+		pr_err("[skip,no-memfd%s]",
+		       (memfd_flags & MFD_HUGETLB) ? " (need hugepages)" : "");
 		goto err_close_dev;
 	}
 
@@ -168,6 +177,11 @@ static struct memory_buffer *udmabuf_alloc(size_t size)
 		goto err_close_memfd;
 	}
 
+	if (memfd_flags & MFD_HUGETLB) {
+		size = roundup(size, MB(2));
+		ctx->size = size;
+	}
+
 	ret = ftruncate(ctx->memfd, size);
 	if (ret == -1) {
 		pr_err("[FAIL,memfd-truncate]");
@@ -699,6 +713,8 @@ static int bind_rx_queue(unsigned int ifindex, unsigned int dmabuf_fd,
 	netdev_bind_rx_req_set_ifindex(req, ifindex);
 	netdev_bind_rx_req_set_fd(req, dmabuf_fd);
 	__netdev_bind_rx_req_set_queues(req, queues, n_queue_index);
+	if (rx_buf_size)
+		netdev_bind_rx_req_set_rx_buf_size(req, rx_buf_size);
 
 	rsp = netdev_bind_rx(*ys, req);
 	if (!rsp) {
@@ -1411,7 +1427,7 @@ int main(int argc, char *argv[])
 	int is_server = 0, opt;
 	int ret, err = 1;
 
-	while ((opt = getopt(argc, argv, "Lls:c:p:v:q:t:f:z:n")) != -1) {
+	while ((opt = getopt(argc, argv, "Lls:c:p:v:q:t:f:z:nb:")) != -1) {
 		switch (opt) {
 		case 'L':
 			fail_on_linear = true;
@@ -1446,6 +1462,33 @@ int main(int argc, char *argv[])
 		case 'n':
 			skip_config = 1;
 			break;
+		case 'b': {
+			char *endp;
+			unsigned long val;
+
+			errno = 0;
+			val = strtoul(optarg, &endp, 0);
+			if (errno || endp == optarg || *endp || val == 0 ||
+			    val > UINT32_MAX) {
+				pr_err("invalid rx_buf_size: %s", optarg);
+				return 1;
+			}
+			if (val & (val - 1)) {
+				pr_err("rx_buf_size must be a power of 2");
+				return 1;
+			}
+			if (val < (unsigned long)getpagesize()) {
+				pr_err("rx_buf_size must be >= PAGE_SIZE (%d)",
+				       getpagesize());
+				return 1;
+			}
+			if (val > MB(2)) {
+				pr_err("rx_buf_size > 2 MB not supported");
+				return 1;
+			}
+			rx_buf_size = val;
+			break;
+		}
 		case '?':
 			fprintf(stderr, "unknown option: %c\n", optopt);
 			break;

-- 
2.53.0-Meta

[PATCH net-next 4/4] selftests/net: devmem.py: add check_rx_large_niov

[Bobby Eshleman]

From: Bobby Eshleman <bobbyeshleman@meta.com>

Add a new devmem test case for binding the dmabuf with rx-buf-size=16K.
The test sweeps RX payload sizes straddling the niov boundary to cover
the sub-niov, exact-niov, and multi-niov RX paths.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 tools/testing/selftests/drivers/net/hw/devmem.py   | 12 +++++-
 .../testing/selftests/drivers/net/hw/devmem_lib.py | 46 +++++++++++++++++++++-
 .../testing/selftests/drivers/net/hw/nk_devmem.py  | 11 +++++-
 3 files changed, 63 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/drivers/net/hw/devmem.py b/tools/testing/selftests/drivers/net/hw/devmem.py
index 031cf9905f65..47b54e18e7a6 100755
--- a/tools/testing/selftests/drivers/net/hw/devmem.py
+++ b/tools/testing/selftests/drivers/net/hw/devmem.py
@@ -2,7 +2,8 @@
 # SPDX-License-Identifier: GPL-2.0
 
 from os import path
-from devmem_lib import setup_test, run_rx, run_tx, run_tx_chunks, run_rx_hds
+from devmem_lib import (setup_test, run_rx, run_tx, run_tx_chunks, run_rx_hds,
+                        run_rx_large_niov)
 from lib.py import ksft_run, ksft_exit, ksft_disruptive
 from lib.py import NetDrvEpEnv
 
@@ -30,11 +31,18 @@ def check_rx_hds(cfg) -> None:
     run_rx_hds(cfg)
 
 
+@ksft_disruptive
+def check_rx_large_niov(cfg) -> None:
+    """Run the devmem RX test with rx-buf-size = 16 KiB."""
+    run_rx_large_niov(cfg)
+
+
 def main() -> None:
     """Run the devmem test cases."""
     with NetDrvEpEnv(__file__) as cfg:
         setup_test(cfg, path.abspath(path.dirname(__file__) + "/ncdevmem"))
-        ksft_run([check_rx, check_tx, check_tx_chunks, check_rx_hds],
+        ksft_run([check_rx, check_tx, check_tx_chunks, check_rx_hds,
+                  check_rx_large_niov],
                  args=(cfg,))
     ksft_exit()
 
diff --git a/tools/testing/selftests/drivers/net/hw/devmem_lib.py b/tools/testing/selftests/drivers/net/hw/devmem_lib.py
index 0921ff03eb81..1d9ad3a294c8 100644
--- a/tools/testing/selftests/drivers/net/hw/devmem_lib.py
+++ b/tools/testing/selftests/drivers/net/hw/devmem_lib.py
@@ -8,7 +8,7 @@ from lib.py import (bkg, cmd, defer, ethtool, rand_port, wait_port_listen,
                     NetdevFamily)
 
 
-def require_devmem(cfg):
+def require_devmem(cfg, rx_buf_size=0):
     """Probe ncdevmem on cfg.ifname and SKIP the test if devmem isn't supported."""
     if not hasattr(cfg, "devmem_probed"):
         probe_command = f"{cfg.bin_local} -f {cfg.ifname}"
@@ -18,6 +18,19 @@ def require_devmem(cfg):
     if not cfg.devmem_supported:
         raise KsftSkipEx("Test requires devmem support")
 
+    if rx_buf_size > 0:
+        if not hasattr(cfg, "devmem_rx_buf_size_probed"):
+            cfg.devmem_rx_buf_size_probed = {}
+
+        if rx_buf_size not in cfg.devmem_rx_buf_size_probed:
+            probe_command = f"{cfg.bin_local} -f {cfg.ifname} -b {rx_buf_size}"
+            cfg.devmem_rx_buf_size_probed[rx_buf_size] = \
+                cmd(probe_command, fail=False, shell=True).ret == 0
+
+        if not cfg.devmem_rx_buf_size_probed[rx_buf_size]:
+            raise KsftSkipEx(
+                f"Test requires devmem rx-buf-size={rx_buf_size} support")
+
 
 def configure_nic(cfg):
     """Channels, rings, RSS, queue lease for netkit devmem."""
@@ -76,7 +89,8 @@ def set_flow_rule(cfg, port):
     return int(re.search(r'ID (\d+)', output).group(1))
 
 
-def ncdevmem_rx(cfg, port, verify=True, fail_on_linear=False, flow_steer=False):
+def ncdevmem_rx(cfg, port, verify=True, fail_on_linear=False, flow_steer=False,
+                rx_buf_size=0):
     """Build the ncdevmem RX listener command."""
     if hasattr(cfg, 'netns'):
         flow_rule_id = set_flow_rule(cfg, port)
@@ -96,6 +110,8 @@ def ncdevmem_rx(cfg, port, verify=True, fail_on_linear=False, flow_steer=False):
         extras.append("-v 7")
     if fail_on_linear:
         extras.append("-L")
+    if rx_buf_size > 0:
+        extras.append(f"-b {rx_buf_size}")
 
     parts = [cfg.bin_local, "-l", f"-f {ifname}", f"-s {addr}",
              f"-p {port}", *extras]
@@ -202,6 +218,32 @@ def run_tx_chunks(cfg):
     ksft_eq(socat.stdout.strip(), "hello\nworld")
 
 
+def run_rx_large_niov(cfg):
+    """Run the devmem RX test with a large niov (rx-buf-size > PAGE_SIZE).
+
+    Sweep payload sizes that straddle the niov boundary: below, equal to,
+    and above rx_buf_size, to exercise sub-niov, exact-niov, and multi-niov
+    RX paths.
+    """
+    require_devmem(cfg, rx_buf_size=16384)
+    configure_nic(cfg)
+    netns = getattr(cfg, "netns", None)
+
+    for size in [1024, 4096, 8192, 16384, 32768, 65536]:
+        port = rand_port()
+        socat = socat_send(cfg, port)
+        listen_cmd = ncdevmem_rx(cfg, port,
+                                 flow_steer=not netns,
+                                 rx_buf_size=16384)
+        data_pipe = (f"yes $(echo -e \x01\x02\x03\x04\x05\x06) | "
+                     f"head -c {size} | {socat}")
+        with bkg(listen_cmd, exit_wait=True, ns=netns) as ncdevmem:
+            wait_port_listen(port, proto="tcp", ns=netns)
+            cmd(data_pipe, host=cfg.remote, shell=True)
+        ksft_eq(ncdevmem.ret, 0,
+                f"large-niov failed for payload size {size}")
+
+
 def run_rx_hds(cfg):
     """Run the HDS test by running devmem RX across a segment size sweep."""
     require_devmem(cfg)
diff --git a/tools/testing/selftests/drivers/net/hw/nk_devmem.py b/tools/testing/selftests/drivers/net/hw/nk_devmem.py
index 300ed2a70ab4..7f1867e4ff32 100755
--- a/tools/testing/selftests/drivers/net/hw/nk_devmem.py
+++ b/tools/testing/selftests/drivers/net/hw/nk_devmem.py
@@ -3,7 +3,8 @@
 """Test devmem TCP with netkit."""
 
 import os
-from devmem_lib import setup_test, run_rx, run_tx, run_tx_chunks, run_rx_hds
+from devmem_lib import (setup_test, run_rx, run_tx, run_tx_chunks, run_rx_hds,
+                        run_rx_large_niov)
 from lib.py import ksft_run, ksft_exit, ksft_disruptive
 from lib.py import NetDrvContEnv
 
@@ -31,6 +32,12 @@ def check_nk_rx_hds(cfg) -> None:
     run_rx_hds(cfg)
 
 
+@ksft_disruptive
+def check_nk_rx_large_niov(cfg) -> None:
+    """Run the devmem RX large-niov test through netkit."""
+    run_rx_large_niov(cfg)
+
+
 def main() -> None:
     """Run the netkit devmem test cases."""
     with NetDrvContEnv(__file__, rxqueues=2, primary_rx_redirect=True) as cfg:
@@ -38,7 +45,7 @@ def main() -> None:
                    os.path.join(os.path.dirname(os.path.abspath(__file__)),
                                 "ncdevmem"))
         ksft_run([check_nk_rx, check_nk_tx, check_nk_tx_chunks,
-                  check_nk_rx_hds], args=(cfg,))
+                  check_nk_rx_hds, check_nk_rx_large_niov], args=(cfg,))
     ksft_exit()
 
 

-- 
2.53.0-Meta

Re: [PATCH net-next 2/4] udmabuf: emit one sg entry per pinned folio

[Christian König]

On 6/4/26 02:42, Bobby Eshleman wrote:
> From: Bobby Eshleman <bobbyeshleman@meta.com>
> 
> get_sg_table() emitted one PAGE_SIZE sg entry per page even when the
> underlying folio was larger.
> 
> Instead, walk folios[] and emit one sg entry per folio. When folios
> represent large pages (as is for MFD_HUGETLB), each sg entry is a large
> page. Normal PAGE_SIZE sg tables are unchanged.
> 
> Required by net/core/devmem to support rx-buf-size > PAGE_SIZE with
> udmabuf.

That doesn't explain why this is required.

Please note that accessing the pages/folio of an sg-table returned by DMA-buf is illegal and strictly forbidden!

Regards,
Christian.

> Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
> ---
>  drivers/dma-buf/udmabuf.c | 47 ++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 42 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index 94b8ecb892bb..f28dd3788ada 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -141,26 +141,63 @@ static void vunmap_udmabuf(struct dma_buf *buf, struct iosys_map *map)
>         vm_unmap_ram(map->vaddr, ubuf->pagecount);
>  }
> 
> +/* Return the number of contiguous pages backed by the folio at @i.
> + * A udmabuf may map only part of a folio, or reference the same folio
> + * in multiple non-contiguous runs, so folio_nr_pages() can't be used.
> + */
> +static pgoff_t udmabuf_folio_nr_pages(struct udmabuf *ubuf, pgoff_t i)
> +{
> +       struct folio *f = ubuf->folios[i];
> +       pgoff_t j;
> +
> +       for (j = 1; i + j < ubuf->pagecount; j++) {
> +               if (ubuf->folios[i + j] != f)
> +                       break;
> +               /* Same folio, but not a sequential offset within it. */
> +               if (ubuf->offsets[i + j] != ubuf->offsets[i] + j * PAGE_SIZE)
> +                       break;
> +       }
> +       return j;
> +}
> +
> +/* Count the contiguous folio runs in @ubuf, one sg entry per run. */
> +static unsigned int udmabuf_sg_nents(struct udmabuf *ubuf)
> +{
> +       unsigned int nents = 0;
> +       pgoff_t i;
> +
> +       for (i = 0; i < ubuf->pagecount; i += udmabuf_folio_nr_pages(ubuf, i))
> +               nents++;
> +       return nents;
> +}
> +
>  static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
>                                      enum dma_data_direction direction)
>  {
>         struct udmabuf *ubuf = buf->priv;
> -       struct sg_table *sg;
>         struct scatterlist *sgl;
> -       unsigned int i = 0;
> +       struct sg_table *sg;
> +       pgoff_t i, run;
> +       unsigned int nents;
>         int ret;
> 
> +       nents = udmabuf_sg_nents(ubuf);
> +
>         sg = kzalloc_obj(*sg);
>         if (!sg)
>                 return ERR_PTR(-ENOMEM);
> 
> -       ret = sg_alloc_table(sg, ubuf->pagecount, GFP_KERNEL);
> +       ret = sg_alloc_table(sg, nents, GFP_KERNEL);
>         if (ret < 0)
>                 goto err_alloc;
> 
> -       for_each_sg(sg->sgl, sgl, ubuf->pagecount, i)
> -               sg_set_folio(sgl, ubuf->folios[i], PAGE_SIZE,
> +       sgl = sg->sgl;
> +       for (i = 0; i < ubuf->pagecount; i += run) {
> +               run = udmabuf_folio_nr_pages(ubuf, i);
> +               sg_set_folio(sgl, ubuf->folios[i], run << PAGE_SHIFT,
>                              ubuf->offsets[i]);
> +               sgl = sg_next(sgl);
> +       }
> 
>         ret = dma_map_sgtable(dev, sg, direction, 0);
>         if (ret < 0)
> 
> --
> 2.53.0-Meta
> 

Re: [PATCH net-next 1/4] net: devmem: allow rx-buf-size > PAGE_SIZE per dmabuf binding

[Stanislav Fomichev]

On 06/03, Bobby Eshleman wrote:
> From: Bobby Eshleman <bobbyeshleman@meta.com>
> 
> Every devmem dmabuf binding today hands the page_pool PAGE_SIZE niovs.
> This caps a single RX descriptor at PAGE_SIZE, burning CPU on buffer
> churn for large flows.
> 
> Add a bind-time netlink attribute, NETDEV_A_DMABUF_RX_BUF_SIZE, that
> lets userspace request a larger niov size. The value must be a power of
> two >= PAGE_SIZE.
> 
> Measurements
> ------------
> Setup: kperf in devmem RX/TX cuda mode, 4 flows, 64 MB messages, 60s,
> dctcp, num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov
> size, mlx5.
> 
> CPU Util:
> 
>    niov        net sirq %        net idle %         app sys %        app idle %
>   -----  ----------------  ----------------  ----------------  ----------------
>      4K   62.38 +/-  8.27   33.40 +/-  7.51   54.15 +/- 10.23   43.67 +/- 10.53
>     16K   58.91 +/-  5.35   35.23 +/-  5.88   41.05 +/-  8.87   56.42 +/-  9.24
>     32K   64.12 +/-  0.68   31.09 +/-  1.48   44.54 +/-  3.51   52.63 +/-  3.65
>     64K   54.69 +/-  5.54   39.67 +/-  5.81   35.47 +/-  3.11   61.97 +/-  3.27
> 
> RX app sys % drops ~19% from 4K to 64K.
> 
> Throughput:
> 
>    niov       RX dev Gbps   RX flow avg Gbps
>   -----  ----------------  -----------------
>      4K  300.63 +/- 53.21    75.16 +/- 13.30
>     16K  321.35 +/- 28.20    80.34 +/-  7.05
>     32K  347.63 +/-  2.20    86.91 +/-  0.55
>     64K  332.11 +/- 14.26    83.03 +/-  3.56
> 
> Throughput seems to increase, but the stdev is pretty wide so could just
> be noise.
> 
> kperf support (not yet merged):
> https://github.com/facebookexperimental/kperf/commit/8837577f920876bce6986ec18869ac04439ebcd2
> 
> Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
> ---
>  Documentation/netlink/specs/netdev.yaml |  8 +++++
>  include/uapi/linux/netdev.h             |  1 +
>  net/core/devmem.c                       | 52 +++++++++++++++++++--------------
>  net/core/devmem.h                       | 13 ++++++---
>  net/core/netdev-genl-gen.c              |  5 ++--
>  net/core/netdev-genl.c                  | 18 ++++++++++--
>  tools/include/uapi/linux/netdev.h       |  1 +
>  7 files changed, 68 insertions(+), 30 deletions(-)
> 
> diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> index a1f4c5a561e9..063119907983 100644
> --- a/Documentation/netlink/specs/netdev.yaml
> +++ b/Documentation/netlink/specs/netdev.yaml
> @@ -591,6 +591,13 @@ attribute-sets:
>          type: u32
>          checks:
>            min: 1
> +      -
> +        name: rx-buf-size
> +        doc: |
> +          Size in bytes of each RX buffer the NIC writes into from the bound
> +          dmabuf. Must be a power of two and >= PAGE_SIZE; defaults to
> +          PAGE_SIZE.
> +        type: u32
>  
>  operations:
>    list:
> @@ -805,6 +812,7 @@ operations:
>              - ifindex
>              - fd
>              - queues
> +            - rx-buf-size
>          reply:
>            attributes:
>              - id
> diff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h
> index 7df1056a35fd..180a4ffffd60 100644
> --- a/include/uapi/linux/netdev.h
> +++ b/include/uapi/linux/netdev.h
> @@ -217,6 +217,7 @@ enum {
>  	NETDEV_A_DMABUF_QUEUES,
>  	NETDEV_A_DMABUF_FD,
>  	NETDEV_A_DMABUF_ID,
> +	NETDEV_A_DMABUF_RX_BUF_SIZE,
>  
>  	__NETDEV_A_DMABUF_MAX,
>  	NETDEV_A_DMABUF_MAX = (__NETDEV_A_DMABUF_MAX - 1)
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 957d6b96216b..5a1c0d7984a8 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -46,7 +46,7 @@ static dma_addr_t net_devmem_get_dma_addr(const struct net_iov *niov)
>  
>  	owner = net_devmem_iov_to_chunk_owner(niov);
>  	return owner->base_dma_addr +
> -	       ((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT);
> +	       ((dma_addr_t)net_iov_idx(niov) << owner->binding->niov_shift);
>  }
>  
>  static void net_devmem_dmabuf_binding_release(struct percpu_ref *ref)
> @@ -93,13 +93,14 @@ net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding)
>  	ssize_t offset;
>  	ssize_t index;
>  
> -	dma_addr = gen_pool_alloc_owner(binding->chunk_pool, PAGE_SIZE,
> +	dma_addr = gen_pool_alloc_owner(binding->chunk_pool,
> +					1UL << binding->niov_shift,
>  					(void **)&owner);
>  	if (!dma_addr)
>  		return NULL;
>  
>  	offset = dma_addr - owner->base_dma_addr;
> -	index = offset / PAGE_SIZE;
> +	index = offset >> binding->niov_shift;
>  	niov = &owner->area.niovs[index];
>  
>  	niov->desc.pp_magic = 0;
> @@ -113,12 +114,13 @@ void net_devmem_free_dmabuf(struct net_iov *niov)
>  {
>  	struct net_devmem_dmabuf_binding *binding = net_devmem_iov_binding(niov);
>  	unsigned long dma_addr = net_devmem_get_dma_addr(niov);
> +	size_t niov_size = 1UL << binding->niov_shift;
>  
>  	if (WARN_ON(!gen_pool_has_addr(binding->chunk_pool, dma_addr,
> -				       PAGE_SIZE)))
> +				       niov_size)))
>  		return;
>  
> -	gen_pool_free(binding->chunk_pool, dma_addr, PAGE_SIZE);
> +	gen_pool_free(binding->chunk_pool, dma_addr, niov_size);
>  }
>  
>  void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding)
> @@ -163,6 +165,9 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
>  	u32 xa_idx;
>  	int err;
>  
> +	if (binding->niov_shift != PAGE_SHIFT)
> +		mp_params.rx_page_size = 1U << binding->niov_shift;
> +
>  	err = netif_mp_open_rxq(dev, rxq_idx, &mp_params, extack);
>  	if (err)
>  		return err;
> @@ -184,14 +189,16 @@ struct net_devmem_dmabuf_binding *
>  net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>  		       struct device *dma_dev,
>  		       enum dma_data_direction direction,
> -		       unsigned int dmabuf_fd, struct netdev_nl_sock *priv,
> +		       unsigned int dmabuf_fd, unsigned int niov_shift,
> +		       struct netdev_nl_sock *priv,
>  		       struct netlink_ext_ack *extack)
>  {
>  	struct net_devmem_dmabuf_binding *binding;
> +	size_t niov_size = 1UL << niov_shift;
>  	static u32 id_alloc_next;
> +	unsigned int sg_idx, i;
>  	struct scatterlist *sg;
>  	struct dma_buf *dmabuf;
> -	unsigned int sg_idx, i;
>  	unsigned long virtual;
>  	int err;
>  
> @@ -213,6 +220,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>  
>  	binding->dev = dev;
>  	binding->vdev = vdev;
> +	binding->niov_shift = niov_shift;
>  	xa_init_flags(&binding->bound_rxqs, XA_FLAGS_ALLOC);
>  
>  	err = percpu_ref_init(&binding->ref,
> @@ -248,18 +256,14 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>  			goto err_unmap;
>  		}
>  		binding->tx_vec = kvmalloc_objs(struct net_iov *,
> -						dmabuf->size / PAGE_SIZE);
> +						dmabuf->size >> niov_shift);
>  		if (!binding->tx_vec) {
>  			err = -ENOMEM;
>  			goto err_unmap;
>  		}
>  	}
>  
> -	/* For simplicity we expect to make PAGE_SIZE allocations, but the
> -	 * binding can be much more flexible than that. We may be able to
> -	 * allocate MTU sized chunks here. Leave that for future work...
> -	 */
> -	binding->chunk_pool = gen_pool_create(PAGE_SHIFT,
> +	binding->chunk_pool = gen_pool_create(niov_shift,
>  					      dev_to_node(&dev->dev));
>  	if (!binding->chunk_pool) {
>  		err = -ENOMEM;
> @@ -273,9 +277,11 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>  		size_t len = sg_dma_len(sg);
>  		struct net_iov *niov;
>  
> -		if (!IS_ALIGNED(len, PAGE_SIZE)) {
> +		if (!IS_ALIGNED(dma_addr, niov_size) ||
> +		    !IS_ALIGNED(len, niov_size)) {
>  			err = -EINVAL;
> -			NL_SET_ERR_MSG(extack, "dma-buf SG length must be PAGE_SIZE aligned");
> +			NL_SET_ERR_MSG(extack,
> +				       "dmabuf sg entry not aligned to niov size");

nit: should we NL_SET_ERR_MSG_FMT here and export chunk len and expected
alignment?

>  			goto err_free_chunks;
>  		}
>  
> @@ -288,7 +294,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>  
>  		owner->area.base_virtual = virtual;
>  		owner->base_dma_addr = dma_addr;
> -		owner->area.num_niovs = len / PAGE_SIZE;
> +		owner->area.num_niovs = len >> niov_shift;
>  		owner->binding = binding;
>  
>  		err = gen_pool_add_owner(binding->chunk_pool, dma_addr,
> @@ -313,7 +319,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>  			page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov),
>  						      net_devmem_get_dma_addr(niov));
>  			if (direction == DMA_TO_DEVICE)
> -				binding->tx_vec[owner->area.base_virtual / PAGE_SIZE + i] = niov;
> +				binding->tx_vec[(owner->area.base_virtual >> niov_shift) + i] = niov;
>  		}
>  
>  		virtual += len;
> @@ -430,13 +436,15 @@ struct net_iov *
>  net_devmem_get_niov_at(struct net_devmem_dmabuf_binding *binding,
>  		       size_t virt_addr, size_t *off, size_t *size)
>  {
> +	size_t niov_size = 1UL << binding->niov_shift;
> +
>  	if (virt_addr >= binding->dmabuf->size)
>  		return NULL;
>  
> -	*off = virt_addr % PAGE_SIZE;
> -	*size = PAGE_SIZE - *off;
> +	*off = virt_addr & (niov_size - 1);
> +	*size = niov_size - *off;
>  
> -	return binding->tx_vec[virt_addr / PAGE_SIZE];
> +	return binding->tx_vec[virt_addr >> binding->niov_shift];
>  }
>  
>  /*** "Dmabuf devmem memory provider" ***/
> @@ -454,8 +462,8 @@ int mp_dmabuf_devmem_init(struct page_pool *pool)
>  	pool->dma_sync = false;
>  	pool->dma_sync_for_cpu = false;
>  
> -	if (pool->p.order != 0)
> -		return -E2BIG;
> +	if (pool->p.order != binding->niov_shift - PAGE_SHIFT)
> +		return -EINVAL;

Any specific reason you change E2BIG to EINVAL?

Re: [PATCH net-next 3/4] selftests/net: ncdevmem: add -b option to set rx-buf-size on bind

[Stanislav Fomichev]

On 06/03, Bobby Eshleman wrote:
> From: Bobby Eshleman <bobbyeshleman@meta.com>
> 
> Add -b <bytes> to request a non-default niov size via
> NETDEV_A_DMABUF_RX_BUF_SIZE. When the value exceeds PAGE_SIZE,
> udmabuf_alloc() switches to an MFD_HUGETLB-backed memfd so each 2 MB
> hugepage produces one naturally-aligned sg entry.
> 
> Reject values > 2 MB up front: MFD_HUGETLB + udmabuf can only guarantee
> 2 MB per sg entry (one hugepage), so a larger rx_buf_size would fail the
> per-sg length/alignment check.
> 
> Add CONFIG_HUGETLBFS=y to drivers/net/hw/config so the new path is
> reachable in the CI kernels built for these tests.

I vaguely remember there was also some kernel cmdline argument to
reserve these? Do we need to also do something to NIPA to reserve 2MB
pages for this test? Or was it for 1GB pages?

Re: [PATCH net-next 1/4] net: devmem: allow rx-buf-size > PAGE_SIZE per dmabuf binding

[Bobby Eshleman]

On Fri, Jun 05, 2026 at 08:33:04AM -0700, Stanislav Fomichev wrote:
> On 06/03, Bobby Eshleman wrote:
> > From: Bobby Eshleman <bobbyeshleman@meta.com>
> > 
> > Every devmem dmabuf binding today hands the page_pool PAGE_SIZE niovs.
> > This caps a single RX descriptor at PAGE_SIZE, burning CPU on buffer
> > churn for large flows.
> > 
> > Add a bind-time netlink attribute, NETDEV_A_DMABUF_RX_BUF_SIZE, that
> > lets userspace request a larger niov size. The value must be a power of
> > two >= PAGE_SIZE.
> > 
> > Measurements
> > ------------
> > Setup: kperf in devmem RX/TX cuda mode, 4 flows, 64 MB messages, 60s,
> > dctcp, num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov
> > size, mlx5.
> > 
> > CPU Util:
> > 
> >    niov        net sirq %        net idle %         app sys %        app idle %
> >   -----  ----------------  ----------------  ----------------  ----------------
> >      4K   62.38 +/-  8.27   33.40 +/-  7.51   54.15 +/- 10.23   43.67 +/- 10.53
> >     16K   58.91 +/-  5.35   35.23 +/-  5.88   41.05 +/-  8.87   56.42 +/-  9.24
> >     32K   64.12 +/-  0.68   31.09 +/-  1.48   44.54 +/-  3.51   52.63 +/-  3.65
> >     64K   54.69 +/-  5.54   39.67 +/-  5.81   35.47 +/-  3.11   61.97 +/-  3.27
> > 
> > RX app sys % drops ~19% from 4K to 64K.
> > 
> > Throughput:
> > 
> >    niov       RX dev Gbps   RX flow avg Gbps
> >   -----  ----------------  -----------------
> >      4K  300.63 +/- 53.21    75.16 +/- 13.30
> >     16K  321.35 +/- 28.20    80.34 +/-  7.05
> >     32K  347.63 +/-  2.20    86.91 +/-  0.55
> >     64K  332.11 +/- 14.26    83.03 +/-  3.56
> > 
> > Throughput seems to increase, but the stdev is pretty wide so could just
> > be noise.
> > 
> > kperf support (not yet merged):
> > https://github.com/facebookexperimental/kperf/commit/8837577f920876bce6986ec18869ac04439ebcd2
> > 
> > Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
> > ---
> >  Documentation/netlink/specs/netdev.yaml |  8 +++++
> >  include/uapi/linux/netdev.h             |  1 +
> >  net/core/devmem.c                       | 52 +++++++++++++++++++--------------
> >  net/core/devmem.h                       | 13 ++++++---
> >  net/core/netdev-genl-gen.c              |  5 ++--
> >  net/core/netdev-genl.c                  | 18 ++++++++++--
> >  tools/include/uapi/linux/netdev.h       |  1 +
> >  7 files changed, 68 insertions(+), 30 deletions(-)
> > 
> > diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> > index a1f4c5a561e9..063119907983 100644
> > --- a/Documentation/netlink/specs/netdev.yaml
> > +++ b/Documentation/netlink/specs/netdev.yaml
> > @@ -591,6 +591,13 @@ attribute-sets:
> >          type: u32
> >          checks:
> >            min: 1
> > +      -
> > +        name: rx-buf-size
> > +        doc: |
> > +          Size in bytes of each RX buffer the NIC writes into from the bound
> > +          dmabuf. Must be a power of two and >= PAGE_SIZE; defaults to
> > +          PAGE_SIZE.
> > +        type: u32
> >  
> >  operations:
> >    list:
> > @@ -805,6 +812,7 @@ operations:
> >              - ifindex
> >              - fd
> >              - queues
> > +            - rx-buf-size
> >          reply:
> >            attributes:
> >              - id
> > diff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h
> > index 7df1056a35fd..180a4ffffd60 100644
> > --- a/include/uapi/linux/netdev.h
> > +++ b/include/uapi/linux/netdev.h
> > @@ -217,6 +217,7 @@ enum {
> >  	NETDEV_A_DMABUF_QUEUES,
> >  	NETDEV_A_DMABUF_FD,
> >  	NETDEV_A_DMABUF_ID,
> > +	NETDEV_A_DMABUF_RX_BUF_SIZE,
> >  
> >  	__NETDEV_A_DMABUF_MAX,
> >  	NETDEV_A_DMABUF_MAX = (__NETDEV_A_DMABUF_MAX - 1)
> > diff --git a/net/core/devmem.c b/net/core/devmem.c
> > index 957d6b96216b..5a1c0d7984a8 100644
> > --- a/net/core/devmem.c
> > +++ b/net/core/devmem.c
> > @@ -46,7 +46,7 @@ static dma_addr_t net_devmem_get_dma_addr(const struct net_iov *niov)
> >  
> >  	owner = net_devmem_iov_to_chunk_owner(niov);
> >  	return owner->base_dma_addr +
> > -	       ((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT);
> > +	       ((dma_addr_t)net_iov_idx(niov) << owner->binding->niov_shift);
> >  }
> >  
> >  static void net_devmem_dmabuf_binding_release(struct percpu_ref *ref)
> > @@ -93,13 +93,14 @@ net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding)
> >  	ssize_t offset;
> >  	ssize_t index;
> >  
> > -	dma_addr = gen_pool_alloc_owner(binding->chunk_pool, PAGE_SIZE,
> > +	dma_addr = gen_pool_alloc_owner(binding->chunk_pool,
> > +					1UL << binding->niov_shift,
> >  					(void **)&owner);
> >  	if (!dma_addr)
> >  		return NULL;
> >  
> >  	offset = dma_addr - owner->base_dma_addr;
> > -	index = offset / PAGE_SIZE;
> > +	index = offset >> binding->niov_shift;
> >  	niov = &owner->area.niovs[index];
> >  
> >  	niov->desc.pp_magic = 0;
> > @@ -113,12 +114,13 @@ void net_devmem_free_dmabuf(struct net_iov *niov)
> >  {
> >  	struct net_devmem_dmabuf_binding *binding = net_devmem_iov_binding(niov);
> >  	unsigned long dma_addr = net_devmem_get_dma_addr(niov);
> > +	size_t niov_size = 1UL << binding->niov_shift;
> >  
> >  	if (WARN_ON(!gen_pool_has_addr(binding->chunk_pool, dma_addr,
> > -				       PAGE_SIZE)))
> > +				       niov_size)))
> >  		return;
> >  
> > -	gen_pool_free(binding->chunk_pool, dma_addr, PAGE_SIZE);
> > +	gen_pool_free(binding->chunk_pool, dma_addr, niov_size);
> >  }
> >  
> >  void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding)
> > @@ -163,6 +165,9 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
> >  	u32 xa_idx;
> >  	int err;
> >  
> > +	if (binding->niov_shift != PAGE_SHIFT)
> > +		mp_params.rx_page_size = 1U << binding->niov_shift;
> > +
> >  	err = netif_mp_open_rxq(dev, rxq_idx, &mp_params, extack);
> >  	if (err)
> >  		return err;
> > @@ -184,14 +189,16 @@ struct net_devmem_dmabuf_binding *
> >  net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
> >  		       struct device *dma_dev,
> >  		       enum dma_data_direction direction,
> > -		       unsigned int dmabuf_fd, struct netdev_nl_sock *priv,
> > +		       unsigned int dmabuf_fd, unsigned int niov_shift,
> > +		       struct netdev_nl_sock *priv,
> >  		       struct netlink_ext_ack *extack)
> >  {
> >  	struct net_devmem_dmabuf_binding *binding;
> > +	size_t niov_size = 1UL << niov_shift;
> >  	static u32 id_alloc_next;
> > +	unsigned int sg_idx, i;
> >  	struct scatterlist *sg;
> >  	struct dma_buf *dmabuf;
> > -	unsigned int sg_idx, i;
> >  	unsigned long virtual;
> >  	int err;
> >  
> > @@ -213,6 +220,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
> >  
> >  	binding->dev = dev;
> >  	binding->vdev = vdev;
> > +	binding->niov_shift = niov_shift;
> >  	xa_init_flags(&binding->bound_rxqs, XA_FLAGS_ALLOC);
> >  
> >  	err = percpu_ref_init(&binding->ref,
> > @@ -248,18 +256,14 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
> >  			goto err_unmap;
> >  		}
> >  		binding->tx_vec = kvmalloc_objs(struct net_iov *,
> > -						dmabuf->size / PAGE_SIZE);
> > +						dmabuf->size >> niov_shift);
> >  		if (!binding->tx_vec) {
> >  			err = -ENOMEM;
> >  			goto err_unmap;
> >  		}
> >  	}
> >  
> > -	/* For simplicity we expect to make PAGE_SIZE allocations, but the
> > -	 * binding can be much more flexible than that. We may be able to
> > -	 * allocate MTU sized chunks here. Leave that for future work...
> > -	 */
> > -	binding->chunk_pool = gen_pool_create(PAGE_SHIFT,
> > +	binding->chunk_pool = gen_pool_create(niov_shift,
> >  					      dev_to_node(&dev->dev));
> >  	if (!binding->chunk_pool) {
> >  		err = -ENOMEM;
> > @@ -273,9 +277,11 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
> >  		size_t len = sg_dma_len(sg);
> >  		struct net_iov *niov;
> >  
> > -		if (!IS_ALIGNED(len, PAGE_SIZE)) {
> > +		if (!IS_ALIGNED(dma_addr, niov_size) ||
> > +		    !IS_ALIGNED(len, niov_size)) {
> >  			err = -EINVAL;
> > -			NL_SET_ERR_MSG(extack, "dma-buf SG length must be PAGE_SIZE aligned");
> > +			NL_SET_ERR_MSG(extack,
> > +				       "dmabuf sg entry not aligned to niov size");
> 
> nit: should we NL_SET_ERR_MSG_FMT here and export chunk len and expected
> alignment?

sgtm!

> >  			goto err_free_chunks;
> >  		}
> >  
> > @@ -288,7 +294,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
> >  
> >  		owner->area.base_virtual = virtual;
> >  		owner->base_dma_addr = dma_addr;
> > -		owner->area.num_niovs = len / PAGE_SIZE;
> > +		owner->area.num_niovs = len >> niov_shift;
> >  		owner->binding = binding;
> >  
> >  		err = gen_pool_add_owner(binding->chunk_pool, dma_addr,
> > @@ -313,7 +319,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
> >  			page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov),
> >  						      net_devmem_get_dma_addr(niov));
> >  			if (direction == DMA_TO_DEVICE)
> > -				binding->tx_vec[owner->area.base_virtual / PAGE_SIZE + i] = niov;
> > +				binding->tx_vec[(owner->area.base_virtual >> niov_shift) + i] = niov;
> >  		}
> >  
> >  		virtual += len;
> > @@ -430,13 +436,15 @@ struct net_iov *
> >  net_devmem_get_niov_at(struct net_devmem_dmabuf_binding *binding,
> >  		       size_t virt_addr, size_t *off, size_t *size)
> >  {
> > +	size_t niov_size = 1UL << binding->niov_shift;
> > +
> >  	if (virt_addr >= binding->dmabuf->size)
> >  		return NULL;
> >  
> > -	*off = virt_addr % PAGE_SIZE;
> > -	*size = PAGE_SIZE - *off;
> > +	*off = virt_addr & (niov_size - 1);
> > +	*size = niov_size - *off;
> >  
> > -	return binding->tx_vec[virt_addr / PAGE_SIZE];
> > +	return binding->tx_vec[virt_addr >> binding->niov_shift];
> >  }
> >  
> >  /*** "Dmabuf devmem memory provider" ***/
> > @@ -454,8 +462,8 @@ int mp_dmabuf_devmem_init(struct page_pool *pool)
> >  	pool->dma_sync = false;
> >  	pool->dma_sync_for_cpu = false;
> >  
> > -	if (pool->p.order != 0)
> > -		return -E2BIG;
> > +	if (pool->p.order != binding->niov_shift - PAGE_SHIFT)
> > +		return -EINVAL;
> 
> Any specific reason you change E2BIG to EINVAL?

It seemed to reflect the new conditional more accurately, as in the case
of order < niov_shift the pool order is too small, not too big. TBH, I'm
not sure if that case is actually ever hit though, at least with current
drivers... 

Not married to it, open to go back to E2BIG.

Best,
Bobby

Re: [PATCH net-next 3/4] selftests/net: ncdevmem: add -b option to set rx-buf-size on bind

[Bobby Eshleman]

On Fri, Jun 05, 2026 at 08:35:25AM -0700, Stanislav Fomichev wrote:
> On 06/03, Bobby Eshleman wrote:
> > From: Bobby Eshleman <bobbyeshleman@meta.com>
> > 
> > Add -b <bytes> to request a non-default niov size via
> > NETDEV_A_DMABUF_RX_BUF_SIZE. When the value exceeds PAGE_SIZE,
> > udmabuf_alloc() switches to an MFD_HUGETLB-backed memfd so each 2 MB
> > hugepage produces one naturally-aligned sg entry.
> > 
> > Reject values > 2 MB up front: MFD_HUGETLB + udmabuf can only guarantee
> > 2 MB per sg entry (one hugepage), so a larger rx_buf_size would fail the
> > per-sg length/alignment check.
> > 
> > Add CONFIG_HUGETLBFS=y to drivers/net/hw/config so the new path is
> > reachable in the CI kernels built for these tests.
> 
> I vaguely remember there was also some kernel cmdline argument to
> reserve these? Do we need to also do something to NIPA to reserve 2MB
> pages for this test? Or was it for 1GB pages?

Good call, my little runner script when prototyping this adds these
pages at runtime and I forgot to move it into the patch, we can have the
py runner do this setup.

Best,
Bobby