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    Subject[PATCH 3.2 196/221] macvtap: limit head length of skb allocated
    3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Jason Wang <jasowang@redhat.com>

    commit 16a3fa28630331e28208872fa5341ce210b901c7 upstream.

    We currently use hdr_len as a hint of head length which is advertised by
    guest. But when guest advertise a very big value, it can lead to an 64K+
    allocating of kmalloc() which has a very high possibility of failure when host
    memory is fragmented or under heavy stress. The huge hdr_len also reduce the
    effect of zerocopy or even disable if a gso skb is linearized in guest.

    To solves those issues, this patch introduces an upper limit (PAGE_SIZE) of the
    head, which guarantees an order 0 allocation each time.

    Cc: Stefan Hajnoczi <stefanha@redhat.com>
    Cc: Michael S. Tsirkin <mst@redhat.com>
    Signed-off-by: Jason Wang <jasowang@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    drivers/net/macvtap.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- a/drivers/net/macvtap.c
    +++ b/drivers/net/macvtap.c
    @@ -672,6 +672,7 @@ static ssize_t macvtap_get_user(struct m
    const struct iovec *iv, unsigned long total_len,
    size_t count, int noblock)
    {
    + int good_linear = SKB_MAX_HEAD(NET_IP_ALIGN);
    struct sk_buff *skb;
    struct macvlan_dev *vlan;
    unsigned long len = total_len;
    @@ -714,6 +715,8 @@ static ssize_t macvtap_get_user(struct m

    if (m && m->msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
    copylen = vnet_hdr.hdr_len ? vnet_hdr.hdr_len : GOODCOPY_LEN;
    + if (copylen > good_linear)
    + copylen = good_linear;
    linear = copylen;
    if (iov_pages(iv, vnet_hdr_len + copylen, count)
    <= MAX_SKB_FRAGS)
    @@ -722,7 +725,10 @@ static ssize_t macvtap_get_user(struct m

    if (!zerocopy) {
    copylen = len;
    - linear = vnet_hdr.hdr_len;
    + if (vnet_hdr.hdr_len > good_linear)
    + linear = good_linear;
    + else
    + linear = vnet_hdr.hdr_len;
    }

    skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, copylen,


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