[PATCH 1/2 net-next] af_unix: improve STREAM behavior with fragmented memory

Eric Dumazet
2013-08-08T21:37:35Z

From: Eric Dumazet <edumazet@google.com>

unix_stream_sendmsg() currently uses order-2 allocations,
and we had numerous reports this can fail.

The __GFP_REPEAT flag present in sock_alloc_send_pskb() is
not helping.

This patch extends the work done in commit eb6a24816b247c
("af_unix: reduce high order page allocations) for
datagram sockets.

This opens the possibility of zero copy IO (splice() and
friends)

The trick is to not use skb_pull() anymore in recvmsg() path,
and instead add a @consumed field in UNIXCB() to track amount
of already read payload in the skb.

There is a performance regression for large sends
because of extra page allocations that will be addressed
in a follow-up patch, allowing sock_alloc_send_pskb()
to attempt high order page allocations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Rientjes <rientjes@google.com>
---
Google-Bug-Id: 9198684

include/net/af_unix.h | 1
net/unix/af_unix.c | 65 ++++++++++++++++++----------------------
2 files changed, 31 insertions(+), 35 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 05442df..a175ba4 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -35,6 +35,7 @@ struct unix_skb_parms {
#ifdef CONFIG_SECURITY_NETWORK
u32 secid; /* Security ID */
#endif
+ u32 consumed;
};

#define UNIXCB(skb) (*(struct unix_skb_parms *)&((skb)->cb))
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index c4ce243..99dc760 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1596,6 +1596,10 @@ out:
return err;
}

+/* We use paged skbs for stream sockets, and limit occupancy to 32768
+ * bytes, and a minimun of a full page.
+ */
+#define UNIX_SKB_FRAGS_SZ (PAGE_SIZE << get_order(32768))

static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
struct msghdr *msg, size_t len)
@@ -1609,6 +1613,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
struct scm_cookie tmp_scm;
bool fds_sent = false;
int max_level;
+ int data_len;

if (NULL == siocb->scm)
siocb->scm = &tmp_scm;
@@ -1635,40 +1640,21 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
goto pipe_err;

while (sent < len) {
- /*
- * Optimisation for the fact that under 0.01% of X
- * messages typically need breaking up.
- */
-
- size = len-sent;
+ size = len - sent;

/* Keep two messages in the pipe so it schedules better */
- if (size > ((sk->sk_sndbuf >> 1) - 64))
- size = (sk->sk_sndbuf >> 1) - 64;
+ size = min_t(int, size, (sk->sk_sndbuf >> 1) - 64);

- if (size > SKB_MAX_ALLOC)
- size = SKB_MAX_ALLOC;
-
- /*
- * Grab a buffer
- */
+ /* allow fallback to order-0 allocations */
+ size = min_t(int, size, SKB_MAX_HEAD(0) + UNIX_SKB_FRAGS_SZ);

- skb = sock_alloc_send_skb(sk, size, msg->msg_flags&MSG_DONTWAIT,
- &err);
+ data_len = max_t(int, 0, size - SKB_MAX_HEAD(0));

- if (skb == NULL)
+ skb = sock_alloc_send_pskb(sk, size - data_len, data_len,
+ msg->msg_flags & MSG_DONTWAIT, &err);
+ if (!skb)
goto out_err;

- /*
- * If you pass two values to the sock_alloc_send_skb
- * it tries to grab the large buffer with GFP_NOFS
- * (which can fail easily), and if it fails grab the
- * fallback size buffer which is under a page and will
- * succeed. [Alan]
- */
- size = min_t(int, size, skb_tailroom(skb));
-
-
/* Only send the fds in the first buffer */
err = unix_scm_to_skb(siocb->scm, skb, !fds_sent);
if (err < 0) {
@@ -1678,7 +1664,10 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
max_level = err + 1;
fds_sent = true;

- err = memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size);
+ skb_put(skb, size - data_len);
+ skb->data_len = data_len;
+ skb->len = size;
+ err = skb_copy_datagram_from_iovec(skb, 0, msg->msg_iov, 0, size);
if (err) {
kfree_skb(skb);
goto out_err;
@@ -1890,6 +1879,11 @@ static long unix_stream_data_wait(struct sock *sk, long timeo,
return timeo;
}

+static unsigned int unix_skb_len(const struct sk_buff *skb)
+{
+ return skb->len - UNIXCB(skb).consumed;
+}
+
static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
struct msghdr *msg, size_t size,
int flags)
@@ -1977,8 +1971,8 @@ again:
}

skip = sk_peek_offset(sk, flags);
- while (skip >= skb->len) {
- skip -= skb->len;
+ while (skip >= unix_skb_len(skb)) {
+ skip -= unix_skb_len(skb);
last = skb;
skb = skb_peek_next(skb, &sk->sk_receive_queue);
if (!skb)
@@ -2005,8 +1999,9 @@ again:
sunaddr = NULL;
}

- chunk = min_t(unsigned int, skb->len - skip, size);
- if (memcpy_toiovec(msg->msg_iov, skb->data + skip, chunk)) {
+ chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
+ if (skb_copy_datagram_iovec(skb, UNIXCB(skb).consumed + skip,
+ msg->msg_iov, chunk)) {
if (copied == 0)
copied = -EFAULT;
break;
@@ -2016,14 +2011,14 @@ again:

/* Mark read part of skb as used */
if (!(flags & MSG_PEEK)) {
- skb_pull(skb, chunk);
+ UNIXCB(skb).consumed += chunk;

sk_peek_offset_bwd(sk, chunk);

if (UNIXCB(skb).fp)
unix_detach_fds(siocb->scm, skb);

- if (skb->len)
+ if (unix_skb_len(skb))
break;

skb_unlink(skb, &sk->sk_receive_queue);
@@ -2107,7 +2102,7 @@ long unix_inq_len(struct sock *sk)
if (sk->sk_type == SOCK_STREAM ||
sk->sk_type == SOCK_SEQPACKET) {
skb_queue_walk(&sk->sk_receive_queue, skb)
- amount += skb->len;
+ amount += unix_skb_len(skb);
} else {
skb = skb_peek(&sk->sk_receive_queue);
if (skb)