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Message-ID: <20130708154814.GI8027@zion.uk.xensource.com>
Date: Mon, 8 Jul 2013 16:48:14 +0100
From: Wei Liu <wei.liu2@...rix.com>
To: Jan Beulich <JBeulich@...e.com>
CC: Wei Liu <wei.liu2@...rix.com>, <davem@...emloft.net>,
Ian Campbell <ian.campbell@...rix.com>,
Dion Kant <g.w.kant@...enet.nl>, <xen-devel@...ts.xen.org>,
<netdev@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may need
to happen earlier
On Mon, Jul 08, 2013 at 03:20:26PM +0100, Jan Beulich wrote:
> >>> On 08.07.13 at 11:59, "Jan Beulich" <JBeulich@...e.com> wrote:
> >>>> On 05.07.13 at 16:53, Wei Liu <wei.liu2@...rix.com> wrote:
> >> On Fri, Jul 05, 2013 at 10:32:41AM +0100, Jan Beulich wrote:
> >>> --- a/drivers/net/xen-netfront.c
> >>> +++ b/drivers/net/xen-netfront.c
> >>> @@ -831,6 +831,15 @@ static RING_IDX xennet_fill_frags(struct
> >>> RING_GET_RESPONSE(&np->rx, ++cons);
> >>> skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
> >>>
> >>> + if (nr_frags == MAX_SKB_FRAGS) {
> >>> + unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
> >>> +
> >>> + BUG_ON(pull_to <= skb_headlen(skb));
> >>> + __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
> >>
> >> skb_headlen is in fact "skb->len - skb->data_len". Looking at the
> >> caller code:
> >>
> >> while loop {
> >> skb_shinfo(skb)->frags[0].page_offset = rx->offset;
> >> skb_frag_size_set(&skb_shinfo(skb)->frags[0], rx->status);
> >> skb->data_len = rx->status;
> >>
> >> i = xennet_fill_frags(np, skb, &tmpq);
> >>
> >> /*
> >
> >>
> >> * Truesize is the actual allocation size, even if the
> >
> >>
> >> * allocation is only partially used.
> >
> >>
> >> */
> >> skb->truesize += PAGE_SIZE * skb_shinfo(skb)->nr_frags;
> >> skb->len += skb->data_len;
> >> }
> >>
> >> handle_incoming_packet();
> >>
> >> You seem to be altering the behavior of the original code, because in
> >> your patch the skb->len is incremented before use, while in the original
> >> code (which calls skb_headlen in handle_incoming_packet) the skb->len is
> >> correctly set.
> >
> > Right. So I basically need to keep skb->len up-to-date along with
> > ->data_len. Just handed a patch to Dion with that done; I'll defer
> > sending a v2 for the upstream code until I know the change works
> > for our kernel.
>
> Okay, so with that done (see below) Dion is now seeing the
> WARN_ON_ONCE(delta < len) in skb_try_coalesce() triggering. Of
> course, with it having crashed before, it's hard to tell whether the
> triggering now is an effect of the patch, or just got unmasked by it.
>
I think the previous crash was due to incorrect accounting.
BUG_ON(pull_to <= skb_headlen(skb))
skb_headlen(skb) = skb->len - skb->data_len
In your first patch, skb_headlen(skb) can return a very large number if
skb->len is smaller then skb->data_len (they are both unsigned), because
before calling skb_headlen() skb->data_len is incremented while skb->len
is not.
As for the WARN_ON_ONCE(delte < len), I haven't looked that far as I
discover a problem with your new patch. Please see below comments.
> Looking over the ->truesize handling, I can't see how the change
> here could break things: RX_COPY_THRESHOLD is already
> accounted for by how alloc_skb() gets called, and the increment
> right after the call to xennet_fill_frags() should now be even more
> correct than before (since __pskb_pull_tail() can drop fragments,
> which would then have made this an over-estimation afaict).
>
> That all said with me knowing pretty little about the networking
> code, so I'd appreciate if you could point out anything wrong with
> my idea of how things work. Additionally - is my fundamental (for
> this patch) assumption right that multiple __pskb_pull_tail() call
> are cumulative (i.e. calling it twice with a delta of
> pull_to - skb_headlen(skb) would indeed end up pulling up to
> pull_to, provided there is enough data)?
>
> Jan
>
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -831,10 +831,20 @@ static RING_IDX xennet_fill_frags(struct
> RING_GET_RESPONSE(&np->rx, ++cons);
> skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
>
> + if (nr_frags == MAX_SKB_FRAGS) {
> + unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
> +
> + BUG_ON(pull_to <= skb_headlen(skb));
> + __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
> + nr_frags = shinfo->nr_frags;
> + }
> + BUG_ON(nr_frags >= MAX_SKB_FRAGS);
> +
> __skb_fill_page_desc(skb, nr_frags,
> skb_frag_page(nfrag),
> rx->offset, rx->status);
>
> + skb->len += rx->status;
> skb->data_len += rx->status;
>
> skb_shinfo(nskb)->nr_frags = 0;
> @@ -929,7 +939,8 @@ static int handle_incoming_queue(struct
> while ((skb = __skb_dequeue(rxq)) != NULL) {
> int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
>
> - __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
> + if (pull_to > skb_headlen(skb))
> + __pskb_pull_tail(skb, pull_to - skb_headlen(skb));
>
> /* Ethernet work: Delayed to here as it peeks the header. */
> skb->protocol = eth_type_trans(skb, dev);
> @@ -1014,7 +1025,7 @@ err:
>
> skb_shinfo(skb)->frags[0].page_offset = rx->offset;
> skb_frag_size_set(&skb_shinfo(skb)->frags[0], rx->status);
> - skb->data_len = rx->status;
> + skb->len = skb->data_len = rx->status;
This is not correct. You should not be needing this. Now you lose count
of SKB head len. Try to go without the above line and see if it makes a
difference?
Wei.
>
> i = xennet_fill_frags(np, skb, &tmpq);
>
> @@ -1023,7 +1034,6 @@ err:
> * allocation is only partially used.
> */
> skb->truesize += PAGE_SIZE * skb_shinfo(skb)->nr_frags;
> - skb->len += skb->data_len;
>
> if (rx->flags & XEN_NETRXF_csum_blank)
> skb->ip_summed = CHECKSUM_PARTIAL;
--
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