Re: [Xen-devel] compound skb frag pages appearing in start_xmit

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-10-10 at 13:24 +0100, Sander Eikelenboom wrote:
> Wednesday, October 10, 2012, 12:13:04 PM, you wrote:
> 
> > On Tue, 2012-10-09 at 15:40 +0100, Ian Campbell wrote:
> >> On Tue, 2012-10-09 at 15:27 +0100, Eric Dumazet wrote:
> >> > On Tue, 2012-10-09 at 15:17 +0100, Ian Campbell wrote:
> >> > 
> >> > > Does the higher order pages effectively reduce the number of frags 
> >> > > which
> >> > > are in use? e.g if MAX_SKB_FRAGS is 16, then for order-0 pages you 
> >> > > could
> >> > > have 64K worth of frag data.
> >> > > 
> >> > > If we switch to order-3 pages everywhere then can the skb contain 512K
> >> > > of data, or does the effective maximum number of frags in an skb reduce
> >> > > to 2?
> >> > 
> >> > effective number of frags reduce to 2 or 3
> >> > 
> >> > (We still limit GSO packets to ~63536 bytes)
> >> 
> >> Great! Then I think the fix is more/less trivial...
> 
> > The following seems to work for me.
> 
> But it doesn't seem to work for me ... dmesg attached.

> [  191.777994] ------------[ cut here ]------------
> [  191.784245] kernel BUG at drivers/net/xen-netback/netback.c:481!

Looks like that BUG_ON is a little aggressive. It'll trigger if the data
happens to end on a frame boundary. Hopefully this will fix it for you:

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index d747e30..f2d6b78 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -477,7 +477,7 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct 
sk_buff *skb,
                size -= bytes;
 
                /* Next frame */
-               if (offset == PAGE_SIZE) {
+               if (offset == PAGE_SIZE && size) {
                        BUG_ON(!PageCompound(page));
                        page++;
                        offset = 0;



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