[ 075/167] [PATCH 15/26] tcp: avoid order-1 allocations on wifi and tx path

Ben Hutchings
2012-05-09T05:59:56Z

From: Eric Dumazet <eric.dumazet@gmail.com>

[ This combines upstream commit
a21d45726acacc963d8baddf74607d9b74e2b723 and the follow-on bug fix
commit 22b4a4f22da4b39c6f7f679fd35f3d35c91bf851 ]

Marc Merlin reported many order-1 allocations failures in TX path on its
wireless setup, that dont make any sense with MTU=1500 network, and non
SG capable hardware.

After investigation, it turns out TCP uses sk_stream_alloc_skb() and
used as a convention skb_tailroom(skb) to know how many bytes of data
payload could be put in this skb (for non SG capable devices)

Note : these skb used kmalloc-4096 (MTU=1500 + MAX_HEADER +
sizeof(struct skb_shared_info) being above 2048)

Later, mac80211 layer need to add some bytes at the tail of skb
(IEEE80211_ENCRYPT_TAILROOM = 18 bytes) and since no more tailroom is
available has to call pskb_expand_head() and request order-1
allocations.

This patch changes sk_stream_alloc_skb() so that only
sk->sk_prot->max_header bytes of headroom are reserved, and use a new
skb field, avail_size to hold the data payload limit.

This way, order-0 allocations done by TCP stack can leave more than 2 KB
of tailroom and no more allocation is performed in mac80211 layer (or
any layer needing some tailroom)

avail_size is unioned with mark/dropcount, since mark will be set later
in IP stack for output packets. Therefore, skb size is unchanged.

Reported-by: Marc MERLIN <marc@merlins.org>
Tested-by: Marc MERLIN <marc@merlins.org>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Correct commit hash for follow-on bug fix]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
include/linux/skbuff.h | 13 +++++++++++++
net/ipv4/tcp.c | 8 ++++----
net/ipv4/tcp_output.c | 3 ++-
3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6cf8b53..e689b47 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -458,6 +458,7 @@ struct sk_buff {
union {
__u32 mark;
__u32 dropcount;
+ __u32 avail_size;
};

__u16 vlan_tci;
@@ -1326,6 +1327,18 @@ static inline int skb_tailroom(const struct sk_buff *skb)
}

/**
+ * skb_availroom - bytes at buffer end
+ * @skb: buffer to check
+ *
+ * Return the number of bytes of free space at the tail of an sk_buff
+ * allocated by sk_stream_alloc()
+ */
+static inline int skb_availroom(const struct sk_buff *skb)
+{
+ return skb_is_nonlinear(skb) ? 0 : skb->avail_size - skb->len;
+}
+
+/**
* skb_reserve - adjust headroom
* @skb: buffer to alter
* @len: bytes to move
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 36611ab..7904db4 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -701,11 +701,12 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp)
skb = alloc_skb_fclone(size + sk->sk_prot->max_header, gfp);
if (skb) {
if (sk_wmem_schedule(sk, skb->truesize)) {
+ skb_reserve(skb, sk->sk_prot->max_header);
/*
* Make sure that we have exactly size bytes
* available to the caller, no more, no less.
*/
- skb_reserve(skb, skb_tailroom(skb) - size);
+ skb->avail_size = size;
return skb;
}
__kfree_skb(skb);
@@ -995,10 +996,9 @@ new_segment:
copy = seglen;

/* Where to copy to? */
- if (skb_tailroom(skb) > 0) {
+ if (skb_availroom(skb) > 0) {
/* We have some space in skb head. Superb! */
- if (copy > skb_tailroom(skb))
- copy = skb_tailroom(skb);
+ copy = min_t(int, copy, skb_availroom(skb));
err = skb_add_data_nocache(sk, skb, from, copy);
if (err)
goto do_fault;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 7413437..c51dd5b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1096,6 +1096,7 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
eat = min_t(int, len, skb_headlen(skb));
if (eat) {
__skb_pull(skb, eat);
+ skb->avail_size -= eat;
len -= eat;
if (!len)
return;
@@ -2060,7 +2061,7 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
/* Punt if not enough space exists in the first SKB for
* the data in the second
*/
- if (skb->len > skb_tailroom(to))
+ if (skb->len > skb_availroom(to))
break;

if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
--
1.7.10