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SubjectRe: [PATCH net V2 1/2] tuntap: do not zerocopy if iov needs more pages than MAX_SKB_FRAGS
On Fri, Jul 19, 2013 at 08:03:35AM +0300, Michael S. Tsirkin wrote:
> 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 <mst@redhat.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>
>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>

Was this patch dropped?
Any objections to merging it for 3.11 and stable?

> > ---
> > 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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