Re: [PATCH] tcp: avoid order-1 allocations on wifi and tx path

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From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Date: Wed, 11 Apr 2012 08:30:48 +0200

> 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@xxxxxxxxxxx>
> Tested-by: Marc MERLIN <marc@xxxxxxxxxxx>
> Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>

Applied and queued up for -stable, thanks Eric.
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