Messages in this thread | | | Date | Thu, 22 May 2014 15:25:29 -0400 | From | Vlad Yasevich <> | Subject | Re: skbuff truesize incorrect. |
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On 05/22/2014 03:07 PM, Jim Baxter wrote: > Hi, I was hoping you can help me with some questions. > > I have been investigating a network issue with bursts of network traffic > over USB CDC-NCM, the issue is that the kernel is dropping packets > because sk_rcvqueues_full() returns true due to skb2->truesize is always > 32960 instead of SKB_TRUESIZE(skb2->len) which is about 1800. > > The code I am trying to fix is this code below, it is splitting a set of > multiple network packets compressed into a single 16k packet into > individual skb's and sending them up the network stack. > > skb2 = skb_clone(skb, GFP_ATOMIC); > if (skb2 == NULL) > goto err; > > if (!skb_pull(skb2, index)) { > ret = -EOVERFLOW; > goto err; > }
This assumes that you original 16K packet is linear. Is that always the case?
> > skb_trim(skb2, dg_len - crc_len); > > My questions are: > > 1) Which buffer size does truesize represent, is it the total buffer or > just the data related to the relevant skb?
Total buffer size (including the struct sk_buff).
> > 2) If truesize is for the skb it is contained within should it be > updated during the call to skb_trim?
No, because the the buffer/memory is still there. skb_trim just sets the tail pointer and adjusts skb->len.
> > 3) Why does the truesize default to 32960?
Probably because that's how much buffer space was actually allocated for the original skb. When you cloned it, you inherited the truesize since a clone is just the struct sk_buff that points into the data of the original.
This is the very same problem that I ran into with SCTP since it has similar code in it. You can play games with truesize manually, but you have to be very careful here.
-vlad > > > Thank you for any help, > Jim Baxter. > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >
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