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Message-ID: <20130710193703.GB20453@zion.uk.xensource.com>
Date: Wed, 10 Jul 2013 20:37:03 +0100
From: Wei Liu <wei.liu2@...rix.com>
To: Matt Wilson <msw@...zon.com>
CC: Xi Xiong <xixiong@...zon.com>, Annie Li <annie.li@...cle.com>,
Wei Liu <wei.liu2@...rix.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 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?
Wei.
>
> Wei.
>
> > }
> >
> > --
> > 1.7.4.5
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