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Date: Thu, 11 Jul 2013 09:25:26 +0800
From: annie li <annie.li@...cle.com>
To: Wei Liu <wei.liu2@...rix.com>
CC: Matt Wilson <msw@...zon.com>, Xi Xiong <xixiong@...zon.com>,
Ian Campbell <Ian.Campbell@...rix.com>, netdev@...r.kernel.org,
xen-devel@...ts.xenproject.org
Subject: Re: [PATCH RFC] xen-netback: calculate the number of slots required
for large MTU vifs
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 <xixiong@...zon.com>
>>>
>>> [ 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 <xixiong@...zon.com>
>>> Reviewed-by: Matt Wilson <msw@...zon.com>
>>> [ msw: minor code cleanups, rewrote commit message, adjusted code
>>> to count RX slots instead of meta structures ]
>>> Signed-off-by: Matt Wilson <msw@...zon.com>
>>> Cc: Annie Li <annie.li@...cle.com>
>>> Cc: Wei Liu <wei.liu2@...rix.com>
>>> Cc: Ian Campbell <Ian.Campbell@...rix.com>
>>> Cc: netdev@...r.kernel.org
>>> Cc: xen-devel@...ts.xenproject.org
>>> ---
>>> 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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