Merge branch 'netlink-mmap-remove'

This file documents how to use memory mapped I/O with netlink.

Author: Patrick McHardy <kaber@trash.net>

Overview

--------

Memory mapped netlink I/O can be used to increase throughput and decrease

overhead of unicast receive and transmit operations. Some netlink subsystems

require high throughput, these are mainly the netfilter subsystems

nfnetlink_queue and nfnetlink_log, but it can also help speed up large

dump operations of f.i. the routing database.

Memory mapped netlink I/O used two circular ring buffers for RX and TX which

are mapped into the processes address space.

The RX ring is used by the kernel to directly construct netlink messages into

user-space memory without copying them as done with regular socket I/O,

additionally as long as the ring contains messages no recvmsg() or poll()

syscalls have to be issued by user-space to get more message.

The TX ring is used to process messages directly from user-space memory, the

kernel processes all messages contained in the ring using a single sendmsg()

call.

Usage overview

--------------

In order to use memory mapped netlink I/O, user-space needs three main changes:

- ring setup

- conversion of the RX path to get messages from the ring instead of recvmsg()

- conversion of the TX path to construct messages into the ring

Ring setup is done using setsockopt() to provide the ring parameters to the

kernel, then a call to mmap() to map the ring into the processes address space:

- setsockopt(fd, SOL_NETLINK, NETLINK_RX_RING, &params, sizeof(params));

- setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, &params, sizeof(params));

- ring = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)

Usage of either ring is optional, but even if only the RX ring is used the

mapping still needs to be writable in order to update the frame status after

processing.

Conversion of the reception path involves calling poll() on the file

descriptor, once the socket is readable the frames from the ring are

processed in order until no more messages are available, as indicated by

a status word in the frame header.

On kernel side, in order to make use of memory mapped I/O on receive, the

originating netlink subsystem needs to support memory mapped I/O, otherwise

it will use an allocated socket buffer as usual and the contents will be

 copied to the ring on transmission, nullifying most of the performance gains.

Dumps of kernel databases automatically support memory mapped I/O.

Conversion of the transmit path involves changing message construction to

use memory from the TX ring instead of (usually) a buffer declared on the

stack and setting up the frame header appropriately. Optionally poll() can

be used to wait for free frames in the TX ring.

Structured and definitions for using memory mapped I/O are contained in

<linux/netlink.h>.

RX and TX rings

----------------

Each ring contains a number of continuous memory blocks, containing frames of

fixed size dependent on the parameters used for ring setup.

Ring:	[ block 0 ]

		[ frame 0 ]

		[ frame 1 ]

	[ block 1 ]

		[ frame 2 ]

		[ frame 3 ]

	...

	[ block n ]

		[ frame 2 * n ]

		[ frame 2 * n + 1 ]

The blocks are only visible to the kernel, from the point of view of user-space

the ring just contains the frames in a continuous memory zone.

The ring parameters used for setting up the ring are defined as follows:

struct nl_mmap_req {

	unsigned int	nm_block_size;

	unsigned int	nm_block_nr;

	unsigned int	nm_frame_size;

	unsigned int	nm_frame_nr;

};

Frames are grouped into blocks, where each block is a continuous region of memory

and holds nm_block_size / nm_frame_size frames. The total number of frames in

the ring is nm_frame_nr. The following invariants hold:

- frames_per_block = nm_block_size / nm_frame_size

- nm_frame_nr = frames_per_block * nm_block_nr

Some parameters are constrained, specifically:

- nm_block_size must be a multiple of the architectures memory page size.

  The getpagesize() function can be used to get the page size.

- nm_frame_size must be equal or larger to NL_MMAP_HDRLEN, IOW a frame must be

  able to hold at least the frame header

- nm_frame_size must be smaller or equal to nm_block_size

- nm_frame_size must be a multiple of NL_MMAP_MSG_ALIGNMENT

- nm_frame_nr must equal the actual number of frames as specified above.

When the kernel can't allocate physically continuous memory for a ring block,

it will fall back to use physically discontinuous memory. This might affect

performance negatively, in order to avoid this the nm_frame_size parameter

should be chosen to be as small as possible for the required frame size and

the number of blocks should be increased instead.

Ring frames

------------

Each frames contain a frame header, consisting of a synchronization word and some

meta-data, and the message itself.

Frame:	[ header message ]

The frame header is defined as follows:

struct nl_mmap_hdr {

	unsigned int	nm_status;

	unsigned int	nm_len;

	__u32		nm_group;

	/* credentials */

	__u32		nm_pid;

	__u32		nm_uid;

	__u32		nm_gid;

};

- nm_status is used for synchronizing processing between the kernel and user-

  space and specifies ownership of the frame as well as the operation to perform

- nm_len contains the length of the message contained in the data area

- nm_group specified the destination multicast group of message

- nm_pid, nm_uid and nm_gid contain the netlink pid, UID and GID of the sending

  process. These values correspond to the data available using SOCK_PASSCRED in

  the SCM_CREDENTIALS cmsg.

The possible values in the status word are:

- NL_MMAP_STATUS_UNUSED:

	RX ring:	frame belongs to the kernel and contains no message

			for user-space. Approriate action is to invoke poll()

			to wait for new messages.

	TX ring:	frame belongs to user-space and can be used for

			message construction.

- NL_MMAP_STATUS_RESERVED:

	RX ring only:	frame is currently used by the kernel for message

			construction and contains no valid message yet.

			Appropriate action is to invoke poll() to wait for

			new messages.

- NL_MMAP_STATUS_VALID:

	RX ring:	frame contains a valid message. Approriate action is

			to process the message and release the frame back to

			the kernel by setting the status to

			NL_MMAP_STATUS_UNUSED or queue the frame by setting the

			status to NL_MMAP_STATUS_SKIP.

	TX ring:	the frame contains a valid message from user-space to

			be processed by the kernel. After completing processing

			the kernel will release the frame back to user-space by

			setting the status to NL_MMAP_STATUS_UNUSED.

- NL_MMAP_STATUS_COPY:

	RX ring only:	a message is ready to be processed but could not be

			stored in the ring, either because it exceeded the

			frame size or because the originating subsystem does

			not support memory mapped I/O. Appropriate action is

			to invoke recvmsg() to receive the message and release

			the frame back to the kernel by setting the status to

			NL_MMAP_STATUS_UNUSED.

- NL_MMAP_STATUS_SKIP:

	RX ring only:	user-space queued the message for later processing, but

			processed some messages following it in the ring. The

			kernel should skip this frame when looking for unused

			frames.

The data area of a frame begins at a offset of NL_MMAP_HDRLEN relative to the

frame header.

TX limitations

--------------

As of Jan 2015 the message is always copied from the ring frame to an

allocated buffer due to unresolved security concerns.

See commit 4682a0358639b29cf ("netlink: Always copy on mmap TX.").

Example

-------

Ring setup:

	unsigned int block_size = 16 * getpagesize();

	struct nl_mmap_req req = {

		.nm_block_size		= block_size,

		.nm_block_nr		= 64,

		.nm_frame_size		= 16384,

		.nm_frame_nr		= 64 * block_size / 16384,

	};

	unsigned int ring_size;

	void *rx_ring, *tx_ring;

	/* Configure ring parameters */

	if (setsockopt(fd, SOL_NETLINK, NETLINK_RX_RING, &req, sizeof(req)) < 0)

		exit(1);

	if (setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, &req, sizeof(req)) < 0)

		exit(1)

	/* Calculate size of each individual ring */

	ring_size = req.nm_block_nr * req.nm_block_size;

	/* Map RX/TX rings. The TX ring is located after the RX ring */

	rx_ring = mmap(NULL, 2 * ring_size, PROT_READ | PROT_WRITE,

		       MAP_SHARED, fd, 0);

	if ((long)rx_ring == -1L)

		exit(1);

	tx_ring = rx_ring + ring_size:

Message reception:

This example assumes some ring parameters of the ring setup are available.

	unsigned int frame_offset = 0;

	struct nl_mmap_hdr *hdr;

	struct nlmsghdr *nlh;

	unsigned char buf[16384];

	ssize_t len;

	while (1) {

		struct pollfd pfds[1];

		pfds[0].fd	= fd;

		pfds[0].events	= POLLIN | POLLERR;

		pfds[0].revents	= 0;

		if (poll(pfds, 1, -1) < 0 && errno != -EINTR)

			exit(1);

		/* Check for errors. Error handling omitted */

		if (pfds[0].revents & POLLERR)

			<handle error>

		/* If no new messages, poll again */

		if (!(pfds[0].revents & POLLIN))

			continue;

		/* Process all frames */

		while (1) {

			/* Get next frame header */

			hdr = rx_ring + frame_offset;

			if (hdr->nm_status == NL_MMAP_STATUS_VALID) {

				/* Regular memory mapped frame */

				nlh = (void *)hdr + NL_MMAP_HDRLEN;

				len = hdr->nm_len;

				/* Release empty message immediately. May happen

				 * on error during message construction.

				 */

				if (len == 0)

					goto release;

			} else if (hdr->nm_status == NL_MMAP_STATUS_COPY) {

				/* Frame queued to socket receive queue */

				len = recv(fd, buf, sizeof(buf), MSG_DONTWAIT);

				if (len <= 0)

					break;

				nlh = buf;

			} else

				/* No more messages to process, continue polling */

				break;

			process_msg(nlh);

release:

			/* Release frame back to the kernel */

			hdr->nm_status = NL_MMAP_STATUS_UNUSED;

			/* Advance frame offset to next frame */

			frame_offset = (frame_offset + frame_size) % ring_size;

		}

	}

Message transmission:

This example assumes some ring parameters of the ring setup are available.

A single message is constructed and transmitted, to send multiple messages

at once they would be constructed in consecutive frames before a final call

to sendto().

	unsigned int frame_offset = 0;

	struct nl_mmap_hdr *hdr;

	struct nlmsghdr *nlh;

	struct sockaddr_nl addr = {

		.nl_family	= AF_NETLINK,

	};

	hdr = tx_ring + frame_offset;

	if (hdr->nm_status != NL_MMAP_STATUS_UNUSED)

		/* No frame available. Use poll() to avoid. */

		exit(1);

	nlh = (void *)hdr + NL_MMAP_HDRLEN;

	/* Build message */

	build_message(nlh);

	/* Fill frame header: length and status need to be set */

	hdr->nm_len	= nlh->nlmsg_len;

	hdr->nm_status	= NL_MMAP_STATUS_VALID;

	if (sendto(fd, NULL, 0, 0, &addr, sizeof(addr)) < 0)

		exit(1);

	/* Advance frame offset to next frame */

	frame_offset = (frame_offset + frame_size) % ring_size;

int nfnetlink_subsys_unregister(const struct nfnetlink_subsystem *n);

int nfnetlink_has_listeners(struct net *net, unsigned int group);

struct sk_buff *nfnetlink_alloc_skb(struct net *net, unsigned int size,

				    u32 dst_portid, gfp_t gfp_mask);

int nfnetlink_send(struct sk_buff *skb, struct net *net, u32 portid,

		   unsigned int group, int echo, gfp_t flags);

int nfnetlink_set_err(struct net *net, u32 portid, u32 group, int error);

extern void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err);

extern int netlink_has_listeners(struct sock *sk, unsigned int group);

extern struct sk_buff *__netlink_alloc_skb(struct sock *ssk, unsigned int size,

					   unsigned int ldiff, u32 dst_portid,

					   gfp_t gfp_mask);

static inline struct sk_buff *

netlink_alloc_skb(struct sock *ssk, unsigned int size, u32 dst_portid,

		  gfp_t gfp_mask)

{

	return __netlink_alloc_skb(ssk, size, 0, dst_portid, gfp_mask);

}

extern int netlink_unicast(struct sock *ssk, struct sk_buff *skb, __u32 portid, int nonblock);

extern int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, __u32 portid,

			     __u32 group, gfp_t allocation);

 * @attrs: netlink attributes

 * @_net: network namespace

 * @user_ptr: user pointers

 * @dst_sk: destination socket

 */

struct genl_info {

	u32			snd_seq;

	struct nlattr **	attrs;

	possible_net_t		_net;

	void *			user_ptr[2];

	struct sock *		dst_sk;

};

static inline struct net *genl_info_net(struct genl_info *info)

void genl_notify(struct genl_family *family, struct sk_buff *skb,

		 struct genl_info *info, u32 group, gfp_t flags);

struct sk_buff *genlmsg_new_unicast(size_t payload, struct genl_info *info,

				    gfp_t flags);

void *genlmsg_put(struct sk_buff *skb, u32 portid, u32 seq,

		  struct genl_family *family, int flags, u8 cmd);

#define NETLINK_PKTINFO			3

#define NETLINK_BROADCAST_ERROR		4

#define NETLINK_NO_ENOBUFS		5

#ifndef __KERNEL__

#define NETLINK_RX_RING			6

#define NETLINK_TX_RING			7

#endif

#define NETLINK_LISTEN_ALL_NSID		8

#define NETLINK_LIST_MEMBERSHIPS	9

#define NETLINK_CAP_ACK			10

	__u32		nm_gid;

};

#ifndef __KERNEL__

enum nl_mmap_status {

	NL_MMAP_STATUS_UNUSED,

	NL_MMAP_STATUS_RESERVED,

#define NL_MMAP_MSG_ALIGNMENT		NLMSG_ALIGNTO

#define NL_MMAP_MSG_ALIGN(sz)		__ALIGN_KERNEL(sz, NL_MMAP_MSG_ALIGNMENT)

#define NL_MMAP_HDRLEN			NL_MMAP_MSG_ALIGN(sizeof(struct nl_mmap_hdr))

#endif

#define NET_MAJOR 36		/* Major 36 is reserved for networking 						*/