Re: [Xen-devel] [PATCH] xen-netback: count number required slots for an skb more carefully

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Sep 03, 2013 at 06:29:50PM +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> When a VM is providing an iSCSI target and the LUN is used by the
> backend domain, the generated skbs for direct I/O writes to the disk
> have large, multi-page skb->data but no frags.
> 
> With some lengths and starting offsets, xen_netbk_count_skb_slots()
> would be one short because the simple calculation of
> DIV_ROUND_UP(skb_headlen(), PAGE_SIZE) was not accounting for the
> decisions made by start_new_rx_buffer() which does not guarantee
> responses are fully packed.
> 
> For example, a skb with length < 2 pages but which spans 3 pages would
> be counted as requiring 2 slots but would actually use 3 slots.
> 
> skb->data:
> 
>     |        1111|222222222222|3333        |
> 
> Fully packed, this would need 2 slots:
> 
>     |111122222222|22223333    |
> 
> But because the 2nd page wholy fits into a slot it is not split across
> slots and goes into a slot of its own:
> 
>     |1111        |222222222222|3333        |
> 
> Miscounting the number of slots means netback may push more responses
> than the number of available requests.  This will cause the frontend
> to get very confused and report "Too many frags/slots".  The frontend
> never recovers and will eventually BUG.
> 
> Fix this by counting the number of required slots more carefully.  In
> xen_netbk_count_skb_slots(), more closely follow the algorithm used by
> xen_netbk_gop_skb() by introducing xen_netbk_count_frag_slots() which
> is the dry-run equivalent of netbk_gop_frag_copy().
> 

Phew! So this is backend miscounting bug. I thought it was a frontend
bug so it didn't ring a bell when we had our face-to-face discussion,
sorry. :-(

This bug was discussed back in July among Annie, Matt, Ian and I. We
finally agreed to take Matt's solution. Matt agreed to post final
version within a week but obviously he's too busy to do so. I was away
so I didn't follow closely. Eventually it fell through the crack. :-(

Matt, do you fancy sending the final version? IIRC the commit message
needs to be re-written. I personally still prefer Matt's solution as
it a) make efficient use of the ring, b) uses ring pointers to
calculate slots which is most accurate, c) removes the dependence on
MAX_SKB_FRAGS in guest RX path.

Anyway, we should get this fixed ASAP.

Thanks
Wei.

REF:
<1373409659-22383-1-git-send-email-msw@xxxxxxxxxx>
<1373350520-19985-1-git-send-email-annie.li@xxxxxxxxxx>


> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
> [Resend to actually Cc netdev, sorry.]
> ---
>  drivers/net/xen-netback/netback.c |   94 
> +++++++++++++++++++++++++------------
>  1 files changed, 64 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c 
> b/drivers/net/xen-netback/netback.c
> index 64828de..f149ce5 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -361,6 +361,49 @@ static bool start_new_rx_buffer(int offset, unsigned 
> long size, int head)
>       return false;
>  }
>  
> +struct xen_netbk_count_slot_state {
> +     unsigned long copy_off;
> +     bool head;
> +};
> +
> +unsigned int xen_netbk_count_frag_slots(struct xenvif *vif,
> +                                     unsigned long offset, unsigned long 
> size,
> +                                     struct xen_netbk_count_slot_state 
> *state)
> +{
> +     unsigned count = 0;
> +
> +     offset &= ~PAGE_MASK;
> +
> +     while (size > 0) {
> +             unsigned long bytes;
> +
> +             bytes = PAGE_SIZE - offset;
> +
> +             if (bytes > size)
> +                     bytes = size;
> +
> +             if (start_new_rx_buffer(state->copy_off, bytes, state->head)) {
> +                     count++;
> +                     state->copy_off = 0;
> +             }
> +
> +             if (state->copy_off + bytes > MAX_BUFFER_OFFSET)
> +                     bytes = MAX_BUFFER_OFFSET - state->copy_off;
> +
> +             state->copy_off += bytes;
> +
> +             offset += bytes;
> +             size -= bytes;
> +
> +             if (offset == PAGE_SIZE)
> +                     offset = 0;
> +
> +             state->head = false;
> +     }
> +
> +     return count;
> +}
> +
>  /*
>   * Figure out how many ring slots we're going to need to send @skb to
>   * the guest. This function is essentially a dry run of
> @@ -368,48 +411,39 @@ static bool start_new_rx_buffer(int offset, unsigned 
> long size, int head)
>   */
>  unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff 
> *skb)
>  {
> +     struct xen_netbk_count_slot_state state;
>       unsigned int count;
> -     int i, copy_off;
> +     unsigned char *data;
> +     unsigned i;
>  
> -     count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> +     state.head = true;
> +     state.copy_off = 0;
>  
> -     copy_off = skb_headlen(skb) % PAGE_SIZE;
> +     /* Slot for the first (partial) page of data. */
> +     count = 1;
>  
> +     /* Need a slot for the GSO prefix for GSO extra data? */
>       if (skb_shinfo(skb)->gso_size)
>               count++;
>  
> -     for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> -             unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
> -             unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
> -             unsigned long bytes;
> -
> -             offset &= ~PAGE_MASK;
> -
> -             while (size > 0) {
> -                     BUG_ON(offset >= PAGE_SIZE);
> -                     BUG_ON(copy_off > MAX_BUFFER_OFFSET);
> -
> -                     bytes = PAGE_SIZE - offset;
> -
> -                     if (bytes > size)
> -                             bytes = size;
> +     data = skb->data;
> +     while (data < skb_tail_pointer(skb)) {
> +             unsigned long offset = offset_in_page(data);
> +             unsigned long size = PAGE_SIZE - offset;
>  
> -                     if (start_new_rx_buffer(copy_off, bytes, 0)) {
> -                             count++;
> -                             copy_off = 0;
> -                     }
> +             if (data + size > skb_tail_pointer(skb))
> +                     size = skb_tail_pointer(skb) - data;
>  
> -                     if (copy_off + bytes > MAX_BUFFER_OFFSET)
> -                             bytes = MAX_BUFFER_OFFSET - copy_off;
> +             count += xen_netbk_count_frag_slots(vif, offset, size, &state);
>  
> -                     copy_off += bytes;
> +             data += size;
> +     }
>  
> -                     offset += bytes;
> -                     size -= bytes;
> +     for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> +             unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
> +             unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
>  
> -                     if (offset == PAGE_SIZE)
> -                             offset = 0;
> -             }
> +             count += xen_netbk_count_frag_slots(vif, offset, size, &state);
>       }
>       return count;
>  }
> -- 
> 1.7.2.5
> 
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