Merge branch 'device-memory-tcp'

Mina Almasry says:

====================
Device Memory TCP

Device memory TCP (devmem TCP) is a proposal for transferring data
to and/or from device memory efficiently, without bouncing the data
to a host memory buffer.

* Problem:

A large amount of data transfers have device memory as the source
and/or destination. Accelerators drastically increased the volume
of such transfers. Some examples include:

- ML accelerators transferring large amounts of training data from storage
  into GPU/TPU memory. In some cases ML training setup time can be as long
  as 50% of TPU compute time, improving data transfer throughput &
  efficiency can help improving GPU/TPU utilization.

- Distributed training, where ML accelerators, such as GPUs on different
  hosts, exchange data among them.

- Distributed raw block storage applications transfer large amounts of
  data with remote SSDs, much of this data does not require host
  processing.

Today, the majority of the Device-to-Device data transfers the network
are implemented as the following low level operations: Device-to-Host
copy, Host-to-Host network transfer, and Host-to-Device copy.

The implementation is suboptimal, especially for bulk data transfers,
and can put significant strains on system resources, such as host memory
bandwidth, PCIe bandwidth, etc. One important reason behind the current
state is the kernel’s lack of semantics to express device to network
transfers.

* Proposal:

In this patch series we attempt to optimize this use case by implementing
socket APIs that enable the user to:

1. send device memory across the network directly, and
2. receive incoming network packets directly into device memory.

Packet _payloads_ go directly from the NIC to device memory for receive
and from device memory to NIC for transmit.
Packet _headers_ go to/from host memory and are processed by the TCP/IP
stack normally. The NIC _must_ support header split to achieve this.

Advantages:

- Alleviate host memory bandwidth pressure, compared to existing
 network-transfer + device-copy semantics.

- Alleviate PCIe BW pressure, by limiting data transfer to the lowest level
  of the PCIe tree, compared to traditional path which sends data through
  the root complex.

* Patch overview:

** Part 1: netlink API

Gives user ability to bind dma-buf to an RX queue.

** Part 2: scatterlist support

Currently the standard for device memory sharing is DMABUF, which doesn't
generate struct pages. On the other hand, networking stack (skbs, drivers,
and page pool) operate on pages. We have 2 options:

1. Generate struct pages for dmabuf device memory, or,
2. Modify the networking stack to process scatterlist.

Approach #1 was attempted in RFC v1. RFC v2 implements approach #2.

** part 3: page pool support

We piggy back on page pool memory providers proposal:
https://github.com/kuba-moo/linux/tree/pp-providers

It allows the page pool to define a memory provider that provides the
page allocation and freeing. It helps abstract most of the device memory
TCP changes from the driver.

** part 4: support for unreadable skb frags

Page pool iovs are not accessible by the host; we implement changes
throughput the networking stack to correctly handle skbs with unreadable
frags.

** Part 5: recvmsg() APIs

We define user APIs for the user to send and receive device memory.

Not included with this series is the GVE devmem TCP support, just to
simplify the review. Code available here if desired:
https://github.com/mina/linux/tree/tcpdevmem

This series is built on top of net-next with Jakub's pp-providers changes
cherry-picked.

* NIC dependencies:

1. (strict) Devmem TCP require the NIC to support header split, i.e. the
   capability to split incoming packets into a header + payload and to put
   each into a separate buffer. Devmem TCP works by using device memory
   for the packet payload, and host memory for the packet headers.

2. (optional) Devmem TCP works better with flow steering support & RSS
   support, i.e. the NIC's ability to steer flows into certain rx queues.
   This allows the sysadmin to enable devmem TCP on a subset of the rx
   queues, and steer devmem TCP traffic onto these queues and non devmem
   TCP elsewhere.

The NIC I have access to with these properties is the GVE with DQO support
running in Google Cloud, but any NIC that supports these features would
suffice. I may be able to help reviewers bring up devmem TCP on their NICs.

* Testing:

The series includes a udmabuf kselftest that show a simple use case of
devmem TCP and validates the entire data path end to end without
a dependency on a specific dmabuf provider.

** Test Setup

Kernel: net-next with this series and memory provider API cherry-picked
locally.

Hardware: Google Cloud A3 VMs.

NIC: GVE with header split & RSS & flow steering support.
====================

Link: https://patch.msgid.link/20240910171458.219195-1-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

4 files changed, 65 insertions, 8 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 2465bdb6037f..e87b5e488325 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3953,6 +3953,8 @@ u8 dev_xdp_prog_count(struct net_device *dev);
 int dev_xdp_propagate(struct net_device *dev, struct netdev_bpf *bpf);
 u32 dev_xdp_prog_id(struct net_device *dev, enum bpf_xdp_mode mode);
 
+u32 dev_get_min_mp_channel_count(const struct net_device *dev);
+
 int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb);
 int dev_forward_skb(struct net_device *dev, struct sk_buff *skb);
 int dev_forward_skb_nomtu(struct net_device *dev, struct sk_buff *skb);
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 5803eb8a157d..39f1d16f3628 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -827,6 +827,8 @@ enum skb_tstamp_type {
  *	@csum_level: indicates the number of consecutive checksums found in
  *		the packet minus one that have been verified as
  *		CHECKSUM_UNNECESSARY (max 3)
+ *	@unreadable: indicates that at least 1 of the fragments in this skb is
+ *		unreadable.
  *	@dst_pending_confirm: need to confirm neighbour
  *	@decrypted: Decrypted SKB
  *	@slow_gro: state present at GRO time, slower prepare step required
@@ -1008,7 +1010,7 @@ struct sk_buff {
 #if IS_ENABLED(CONFIG_IP_SCTP)
 	__u8			csum_not_inet:1;
 #endif
-
+	__u8			unreadable:1;
 #if defined(CONFIG_NET_SCHED) || defined(CONFIG_NET_XGRESS)
 	__u16			tc_index;	/* traffic control index */
 #endif
@@ -1824,6 +1826,12 @@ static inline void skb_zcopy_downgrade_managed(struct sk_buff *skb)
 		__skb_zcopy_downgrade_managed(skb);
 }
 
+/* Return true if frags in this skb are readable by the host. */
+static inline bool skb_frags_readable(const struct sk_buff *skb)
+{
+	return !skb->unreadable;
+}
+
 static inline void skb_mark_not_on_list(struct sk_buff *skb)
 {
 	skb->next = NULL;
@@ -2540,10 +2548,17 @@ static inline void skb_len_add(struct sk_buff *skb, int delta)
 static inline void __skb_fill_netmem_desc(struct sk_buff *skb, int i,
 					  netmem_ref netmem, int off, int size)
 {
-	struct page *page = netmem_to_page(netmem);
+	struct page *page;
 
 	__skb_fill_netmem_desc_noacc(skb_shinfo(skb), i, netmem, off, size);
 
+	if (netmem_is_net_iov(netmem)) {
+		skb->unreadable = true;
+		return;
+	}
+
+	page = netmem_to_page(netmem);
+
 	/* Propagate page pfmemalloc to the skb if we can. The problem is
 	 * that not all callers have unique ownership of the page but rely
 	 * on page_is_pfmemalloc doing the right thing(tm).
@@ -3524,21 +3539,58 @@ static inline void skb_frag_off_copy(skb_frag_t *fragto,
 	fragto->offset = fragfrom->offset;
 }
 
+/* Return: true if the skb_frag contains a net_iov. */
+static inline bool skb_frag_is_net_iov(const skb_frag_t *frag)
+{
+	return netmem_is_net_iov(frag->netmem);
+}
+
+/**
+ * skb_frag_net_iov - retrieve the net_iov referred to by fragment
+ * @frag: the fragment
+ *
+ * Return: the &struct net_iov associated with @frag. Returns NULL if this
+ * frag has no associated net_iov.
+ */
+static inline struct net_iov *skb_frag_net_iov(const skb_frag_t *frag)
+{
+	if (!skb_frag_is_net_iov(frag))
+		return NULL;
+
+	return netmem_to_net_iov(frag->netmem);
+}
+
 /**
  * skb_frag_page - retrieve the page referred to by a paged fragment
  * @frag: the paged fragment
  *
- * Returns the &struct page associated with @frag.
+ * Return: the &struct page associated with @frag. Returns NULL if this frag
+ * has no associated page.
  */
 static inline struct page *skb_frag_page(const skb_frag_t *frag)
 {
+	if (skb_frag_is_net_iov(frag))
+		return NULL;
+
 	return netmem_to_page(frag->netmem);
 }
 
+/**
+ * skb_frag_netmem - retrieve the netmem referred to by a fragment
+ * @frag: the fragment
+ *
+ * Return: the &netmem_ref associated with @frag.
+ */
+static inline netmem_ref skb_frag_netmem(const skb_frag_t *frag)
+{
+	return frag->netmem;
+}
+
 int skb_pp_cow_data(struct page_pool *pool, struct sk_buff **pskb,
 		    unsigned int headroom);
 int skb_cow_data_for_xdp(struct page_pool *pool, struct sk_buff **pskb,
 			 struct bpf_prog *prog);
+
 /**
  * skb_frag_address - gets the address of the data contained in a paged fragment
  * @frag: the paged fragment buffer
@@ -3548,6 +3600,9 @@ int skb_cow_data_for_xdp(struct page_pool *pool, struct sk_buff **pskb,
  */
 static inline void *skb_frag_address(const skb_frag_t *frag)
 {
+	if (!skb_frag_page(frag))
+		return NULL;
+
 	return page_address(skb_frag_page(frag)) + skb_frag_off(frag);
 }
 
diff --git a/include/linux/skbuff_ref.h b/include/linux/skbuff_ref.h
index 16c241a23472..0f3c58007488 100644
--- a/include/linux/skbuff_ref.h
+++ b/include/linux/skbuff_ref.h
@@ -34,14 +34,13 @@ static inline void skb_frag_ref(struct sk_buff *skb, int f)
 
 bool napi_pp_put_page(netmem_ref netmem);
 
-static inline void
-skb_page_unref(struct page *page, bool recycle)
+static inline void skb_page_unref(netmem_ref netmem, bool recycle)
 {
 #ifdef CONFIG_PAGE_POOL
-	if (recycle && napi_pp_put_page(page_to_netmem(page)))
+	if (recycle && napi_pp_put_page(netmem))
 		return;
 #endif
-	put_page(page);
+	put_page(netmem_to_page(netmem));
 }
 
 /**
@@ -54,7 +53,7 @@ skb_page_unref(struct page *page, bool recycle)
  */
 static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle)
 {
-	skb_page_unref(skb_frag_page(frag), recycle);
+	skb_page_unref(skb_frag_netmem(frag), recycle);
 }
 
 /**
diff --git a/include/linux/socket.h b/include/linux/socket.h
index df9cdb8bbfb8..d18cc47e89bd 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -327,6 +327,7 @@ struct ucred {
 					  * plain text and require encryption
 					  */
 
+#define MSG_SOCK_DEVMEM 0x2000000	/* Receive devmem skbs as cmsg */
 #define MSG_ZEROCOPY	0x4000000	/* Use user data in kernel path */
 #define MSG_SPLICE_PAGES 0x8000000	/* Splice the pages from the iterator in sendmsg() */
 #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */