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
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: default avatarMarc MERLIN <marc@merlins.org>
Tested-by: default avatarMarc MERLIN <marc@merlins.org>
Signed-off-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 87151b86

include/linux/skbuff.h

+13 −0
Original line number Diff line number Diff line
@@ -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

net/ipv4/tcp.c

+4 −4
Original line number Diff line number Diff line
@@ -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;

net/ipv4/tcp_output.c

+1 −1
Original line number Diff line number Diff line
@@ -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))

			break;



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

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