Commit a21d4572 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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tcp: avoid order-1 allocations on wifi and tx path

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

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: default avatarMarc MERLIN <>
Tested-by: default avatarMarc MERLIN <>
Signed-off-by: default avatarEric Dumazet <>
Signed-off-by: default avatarDavid S. Miller <>
parent 87151b86
......@@ -481,6 +481,7 @@ struct sk_buff {
union {
__u32 mark;
__u32 dropcount;
__u32 avail_size;
sk_buff_data_t transport_header;
......@@ -1365,6 +1366,18 @@ static inline int skb_tailroom(const struct sk_buff *skb)
return skb_is_nonlinear(skb) ? 0 : skb->end - skb->tail;
* 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
......@@ -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;
......@@ -995,10 +996,9 @@ int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
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;
......@@ -2060,7 +2060,7 @@ 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))
if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
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