On Thu, Jul 18, 2013 at 10:55:15AM +0800, Jason Wang wrote:
> We try to linearize part of the skb when the number of iov is greater than
> MAX_SKB_FRAGS. This is not enough since each single vector may occupy more
> than
> one pages, so zerocopy_sg_fromiovec() may still fail and may break the guest
> network.
>
> Solve this problem by calculate the pages needed for iov before trying to do
> zerocopy and switch to use copy instead of zerocopy if it needs more than
> MAX_SKB_FRAGS.
>
> This is done through introducing a new helper to count the pages for iov, and
> call uarg->callback() manually when switching from zerocopy to copy to notify
> vhost.
>
> We can do further optimization on top.
>
> The bug were introduced from commit 0690899b4d4501b3505be069b9a687e68ccbe15b
> (tun: experimental zero copy tx support)
>
> Cc: Michael S. Tsirkin <m...@redhat.com>
> Signed-off-by: Jason Wang <jasow...@redhat.com>
Acked-by: Michael S. Tsirkin <m...@redhat.com>
> ---
> The patch is needed fot -stable.
> ---
> drivers/net/tun.c | 62 ++++++++++++++++++++++++++++++++--------------------
> 1 files changed, 38 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 5cdcf92..db690a3 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1035,6 +1035,29 @@ static int zerocopy_sg_from_iovec(struct sk_buff *skb,
> const struct iovec *from,
> return 0;
> }
>
> +static unsigned long iov_pages(const struct iovec *iv, int offset,
> + unsigned long nr_segs)
> +{
> + unsigned long seg, base;
> + int pages = 0, len, size;
> +
> + while (nr_segs && (offset >= iv->iov_len)) {
> + offset -= iv->iov_len;
> + ++iv;
> + --nr_segs;
> + }
> +
> + for (seg = 0; seg < nr_segs; seg++) {
> + base = (unsigned long)iv[seg].iov_base + offset;
> + len = iv[seg].iov_len - offset;
> + size = ((base & ~PAGE_MASK) + len + ~PAGE_MASK) >> PAGE_SHIFT;
> + pages += size;
> + offset = 0;
> + }
> +
> + return pages;
> +}
> +
> /* Get packet from user space buffer */
> static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> void *msg_control, const struct iovec *iv,
> @@ -1082,32 +1105,18 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> return -EINVAL;
> }
>
> - if (msg_control)
> - zerocopy = true;
> -
> - if (zerocopy) {
> - /* Userspace may produce vectors with count greater than
> - * MAX_SKB_FRAGS, so we need to linearize parts of the skb
> - * to let the rest of data to be fit in the frags.
> - */
> - if (count > MAX_SKB_FRAGS) {
> - copylen = iov_length(iv, count - MAX_SKB_FRAGS);
> - if (copylen < offset)
> - copylen = 0;
> - else
> - copylen -= offset;
> - } else
> - copylen = 0;
> - /* There are 256 bytes to be copied in skb, so there is enough
> - * room for skb expand head in case it is used.
> + if (msg_control) {
> + /* There are 256 bytes to be copied in skb, so there is
> + * enough room for skb expand head in case it is used.
> * The rest of the buffer is mapped from userspace.
> */
> - if (copylen < gso.hdr_len)
> - copylen = gso.hdr_len;
> - if (!copylen)
> - copylen = GOODCOPY_LEN;
> + copylen = gso.hdr_len ? gso.hdr_len : GOODCOPY_LEN;
> linear = copylen;
> - } else {
> + if (iov_pages(iv, offset + copylen, count) <= MAX_SKB_FRAGS)
> + zerocopy = true;
> + }
> +
> + if (!zerocopy) {
> copylen = len;
> linear = gso.hdr_len;
> }
> @@ -1121,8 +1130,13 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
>
> if (zerocopy)
> err = zerocopy_sg_from_iovec(skb, iv, offset, count);
> - else
> + else {
> err = skb_copy_datagram_from_iovec(skb, 0, iv, offset, len);
> + if (!err && msg_control) {
> + struct ubuf_info *uarg = msg_control;
> + uarg->callback(uarg, false);
> + }
> + }
>
> if (err) {
> tun->dev->stats.rx_dropped++;
> --
> 1.7.1
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