Subject: Re: [PATCH RFC] xen-netback: calculate the number of slots required for large MTU vifs

From: annie li <[email-masked]>
Date: Thu, 11 Jul 2013 09:25:26 +0800

On 2013-7-11 3:37, Wei Liu wrote:
> On Wed, Jul 10, 2013 at 09:13:33AM +0100, Wei Liu wrote:
>> On Tue, Jul 09, 2013 at 10:40:59PM +0000, Matt Wilson wrote:
>>> From: Xi Xiong <[email-masked]>
>>>
>>> [ note: I've just cherry picked this onto net-next, and only compile
>>>    tested. This a RFC only. -msw ]
>>>
>> Should probably rebase it on net.git because it is a bug fix. Let's
>> worry about that later...
>>
>>> Currently the number of RX slots required to transmit a SKB to
>>> xen-netfront can be miscalculated when an interface uses a MTU larger
>>> than PAGE_SIZE. If the slot calculation is wrong, xen-netback can
>>> pause the queue indefinitely or reuse slots. The former manifests as a
>>> loss of connectivity to the guest (which can be restored by lowering
>>> the MTU set on the interface). The latter manifests with "Bad grant
>>> reference" messages from Xen such as:
>>>
>>> (XEN) grant_table.c:1797:d0 Bad grant reference 264241157
>>>
>>> and kernel messages within the guest such as:
>>>
>>> [  180.419567] net eth0: Invalid extra type: 112
>>> [  180.868620] net eth0: rx->offset: 0, size: 4294967295
>>> [  180.868629] net eth0: rx->offset: 0, size: 4294967295
>>>
>>> BUG_ON() assertions can also be hit if RX slots are exhausted while
>>> handling a SKB.
>>>
>>> This patch changes xen_netbk_rx_action() to count the number of RX
>>> slots actually consumed by netbk_gop_skb() instead of using nr_frags + 1.
>>> This prevents under-counting the number of RX slots consumed when a
>>> SKB has a large linear buffer.
>>>
>>> Additionally, we now store the estimated number of RX slots required
>>> to handle a SKB in the cb overlay. This value is used to determine if
>>> the next SKB in the queue can be processed.
>>>
>>> Finally, the logic in start_new_rx_buffer() can cause RX slots to be
>>> wasted when setting up copy grant table operations for SKBs with large
>>> linear buffers. For example, a SKB with skb_headlen() equal to 8157
>>> bytes that starts 64 bytes 64 bytes from the start of the page will
>> Duplicated "64 bytes". And this change looks like an improvement not a
>> bug fix. Probably submit a separate patch for this?
>>
>>> consume three RX slots instead of two. This patch changes the "head"
>>> parameter to netbk_gop_frag_copy() to act as a flag. When set,
>>> start_new_rx_buffer() will always place as much data as possible into
>>> each RX slot.
>>>
>>> Signed-off-by: Xi Xiong <[email-masked]>
>>> Reviewed-by: Matt Wilson <[email-masked]>
>>> [ msw: minor code cleanups, rewrote commit message, adjusted code
>>>    to count RX slots instead of meta structures ]
>>> Signed-off-by: Matt Wilson <[email-masked]>
>>> Cc: Annie Li <[email-masked]>
>>> Cc: Wei Liu <[email-masked]>
>>> Cc: Ian Campbell <[email-masked]>
>>> Cc: [email-masked]
>>> Cc: [email-masked]
>>> ---
>>>   drivers/net/xen-netback/netback.c |   51 ++++++++++++++++++++++--------------
>>>   1 files changed, 31 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
>>> index 64828de..82dd207 100644
>>> --- a/drivers/net/xen-netback/netback.c
>>> +++ b/drivers/net/xen-netback/netback.c
>>> @@ -110,6 +110,11 @@ union page_ext {
>>>   	void *mapping;
>>>   };
>>>   
>>> +struct skb_cb_overlay {
>>> +	int meta_slots_used;
>>> +	int peek_slots_count;
>>> +};
>>> +
>>>   struct xen_netbk {
>>>   	wait_queue_head_t wq;
>>>   	struct task_struct *task;
>>> @@ -370,6 +375,7 @@ unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
>>>   {
>>>   	unsigned int count;
>>>   	int i, copy_off;
>>> +	struct skb_cb_overlay *sco;
>>>   
>>>   	count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
>>>   
>>> @@ -411,6 +417,9 @@ unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
>>>   				offset = 0;
>>>   		}
>>>   	}
>>> +
>>> +	sco = (struct skb_cb_overlay *) skb->cb;
>>> +	sco->peek_slots_count = count;
>>>   	return count;
>>>   }
>>>   
>>> @@ -443,13 +452,12 @@ static struct netbk_rx_meta *get_next_rx_buffer(struct xenvif *vif,
>>>   }
>>>   
>>>   /*
>>> - * Set up the grant operations for this fragment. If it's a flipping
>>> - * interface, we also set up the unmap request from here.
>>> + * Set up the grant operations for this fragment.
>>>    */
>>>   static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
>>>   				struct netrx_pending_operations *npo,
>>>   				struct page *page, unsigned long size,
>>> -				unsigned long offset, int *head)
>>> +				unsigned long offset, int head, int *first)
>>>   {
>>>   	struct gnttab_copy *copy_gop;
>>>   	struct netbk_rx_meta *meta;
>>> @@ -479,12 +487,12 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
>>>   		if (bytes > size)
>>>   			bytes = size;
>>>   
>>> -		if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
>>> +		if (start_new_rx_buffer(npo->copy_off, bytes, head)) {
>>>   			/*
>>>   			 * Netfront requires there to be some data in the head
>>>   			 * buffer.
>>>   			 */
>>> -			BUG_ON(*head);
>>> +			BUG_ON(*first);
>>>   
>>>   			meta = get_next_rx_buffer(vif, npo);
>>>   		}
>>> @@ -529,10 +537,10 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
>>>   		}
>>>   
>>>   		/* Leave a gap for the GSO descriptor. */
>>> -		if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
>>> +		if (*first && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
>>>   			vif->rx.req_cons++;
>>>   
>>> -		*head = 0; /* There must be something in this buffer now. */
>>> +		*first = 0; /* There must be something in this buffer now. */
>>>   
>>>   	}
>>>   }
>>> @@ -558,7 +566,7 @@ static int netbk_gop_skb(struct sk_buff *skb,
>>>   	struct xen_netif_rx_request *req;
>>>   	struct netbk_rx_meta *meta;
>>>   	unsigned char *data;
>>> -	int head = 1;
>>> +	int first = 1;
>>>   	int old_meta_prod;
>>>   
>>>   	old_meta_prod = npo->meta_prod;
>>> @@ -594,16 +602,16 @@ static int netbk_gop_skb(struct sk_buff *skb,
>>>   			len = skb_tail_pointer(skb) - data;
>>>   
>>>   		netbk_gop_frag_copy(vif, skb, npo,
>>> -				    virt_to_page(data), len, offset, &head);
>>> +				virt_to_page(data), len, offset, 1, &first);
>>>   		data += len;
>>>   	}
>>>   
>>>   	for (i = 0; i < nr_frags; i++) {
>>>   		netbk_gop_frag_copy(vif, skb, npo,
>>> -				    skb_frag_page(&skb_shinfo(skb)->frags[i]),
>>> -				    skb_frag_size(&skb_shinfo(skb)->frags[i]),
>>> -				    skb_shinfo(skb)->frags[i].page_offset,
>>> -				    &head);
>>> +				skb_frag_page(&skb_shinfo(skb)->frags[i]),
>>> +				skb_frag_size(&skb_shinfo(skb)->frags[i]),
>>> +				skb_shinfo(skb)->frags[i].page_offset,
>>> +				0, &first);
>>>   	}
>>>   
>>>   	return npo->meta_prod - old_meta_prod;
>>> @@ -661,10 +669,6 @@ static void netbk_add_frag_responses(struct xenvif *vif, int status,
>>>   	}
>>>   }
>>>   
>>> -struct skb_cb_overlay {
>>> -	int meta_slots_used;
>>> -};
>>> -
>>>   static void xen_netbk_rx_action(struct xen_netbk *netbk)
>>>   {
>>>   	struct xenvif *vif = NULL, *tmp;
>>> @@ -690,19 +694,26 @@ static void xen_netbk_rx_action(struct xen_netbk *netbk)
>>>   	count = 0;
>>>   
>>>   	while ((skb = skb_dequeue(&netbk->rx_queue)) != NULL) {
>>> +		RING_IDX old_rx_req_cons;
>>> +
>>>   		vif = netdev_priv(skb->dev);
>>>   		nr_frags = skb_shinfo(skb)->nr_frags;
>>>   
>>> +		old_rx_req_cons = vif->rx.req_cons;
>>>   		sco = (struct skb_cb_overlay *)skb->cb;
>>>   		sco->meta_slots_used = netbk_gop_skb(skb, &npo);
>>>   
>>> -		count += nr_frags + 1;
>>> +		count += vif->rx.req_cons - old_rx_req_cons;
>>>   
>>>   		__skb_queue_tail(&rxq, skb);
>>>   
>>> +		skb = skb_peek(&netbk->rx_queue);
>>> +		if (skb == NULL)
>>> +			break;
>>> +		sco = (struct skb_cb_overlay *) skb->cb;
>>> +
>>>   		/* Filled the batch queue? */
>>> -		/* XXX FIXME: RX path dependent on MAX_SKB_FRAGS */
>>> -		if (count + MAX_SKB_FRAGS >= XEN_NETIF_RX_RING_SIZE)
>>> +		if (count + sco->peek_slots_count >= XEN_NETIF_RX_RING_SIZE)
>>>   			break;
>> Using req_cons to count is OK, but since you've already store the number
>> of slots in sco->peek_slots_count before actually queueing the packet,
>> why don't you use that directly?
>>
> Ah, I must be off my head here. I mixed several patches 1) my RFC patch
> that removes MAX_SKB_FRAGS in RX path, 2) Annie's patch and 3) your
> patch.
>
> The reason that your patches uses req_cons instead of pre-calculated
> value is, that value returned by xen_netbk_skb_count_slots is actually
> *wrong* -- that's what Annie tried to fix in her patch.
>
> After fixing xen_netbk_skb_count_slots, we would need the above snippet
> (or the snippet I proposed in my RFC patch) to prevent overruning the
> ring.
>
> So a proper patch to this issue (not couting RX slots correctly causing
> overrun of the ring) would be the above two aspects combined.
>
> Comments?

I agree, two patches looks fine.
xen_netbk_skb_count_slots returns wrong value, this could be fixed by my 
patch.
protection from overrunning the ring, this could be implemented by 
Wei's/Matt's patch.

Thanks
Annie
>
>
> Wei.
>
>> Wei.
>>
>>>   	}
>>>   
>>> -- 
>>> 1.7.4.5
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