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  • Eric Dumazet's avatar
    tcp: avoid order-1 allocations on wifi and tx path · a21d4572
    Eric Dumazet authored
    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 <>