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Re: [Xen-devel] [PATCH net] xen-netback: Fix handling of skbs requiring too many slots
Hi,
This patch have made the net and net-next trees already, but not the
mainline 3.15. That's unfortunate, because just 2 days after someone
already spotted the problem (see "netback oops in Linux 3.14 / 3.15" on
xen-devel)
David, should I resend the patch for stable maintaners to include it in
the 3.15 stable branch?
Regards,
Zoli
On 03/06/14 21:30, Zoltan Kiss wrote:
A recent commit (a02eb4 "xen-netback: worse-case estimate in xenvif_rx_action is
underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that triggers
the next BUG_ON a few lines down, as the packet consumes more slots than
estimated.
This patch introduces full_coalesce on the skb callback buffer, which is used in
start_new_rx_buffer() to decide whether netback needs coalescing more
aggresively. By doing that, no packet should need more than
XEN_NETIF_MAX_TX_SIZE / PAGE_SIZE data slots, as the provided buffers are fully
utilized.
Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
---
diff --git a/drivers/net/xen-netback/netback.c
b/drivers/net/xen-netback/netback.c
index 64ab1d1..65fbcd1 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -163,7 +163,10 @@ bool xenvif_rx_ring_slots_available(struct xenvif *vif,
int needed)
* adding 'size' bytes to a buffer which currently contains 'offset'
* bytes.
*/
-static bool start_new_rx_buffer(int offset, unsigned long size, int head)
+static bool start_new_rx_buffer(int offset,
+ unsigned long size,
+ int head,
+ bool full_coalesce)
{
/* simple case: we have completely filled the current buffer. */
if (offset == MAX_BUFFER_OFFSET)
@@ -175,6 +178,7 @@ static bool start_new_rx_buffer(int offset, unsigned long
size, int head)
* (i) this frag would fit completely in the next buffer
* and (ii) there is already some data in the current buffer
* and (iii) this is not the head buffer.
+ * and (iv) there is no need to fully utilize the buffers
*
* Where:
* - (i) stops us splitting a frag into two copies
@@ -185,6 +189,8 @@ static bool start_new_rx_buffer(int offset, unsigned long
size, int head)
* by (ii) but is explicitly checked because
* netfront relies on the first buffer being
* non-empty and can crash otherwise.
+ * - (iv) is needed for skbs which can use up more than MAX_SKB_FRAGS
+ * slot
*
* This means we will effectively linearise small
* frags but do not needlessly split large buffers
@@ -192,7 +198,10 @@ static bool start_new_rx_buffer(int offset, unsigned long
size, int head)
* own buffers as before.
*/
BUG_ON(size > MAX_BUFFER_OFFSET);
- if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
+ if ((offset + size > MAX_BUFFER_OFFSET) &&
+ offset &&
+ !head &&
+ !full_coalesce)
return true;
return false;
@@ -227,6 +236,13 @@ static struct xenvif_rx_meta *get_next_rx_buffer(struct
xenvif *vif,
return meta;
}
+struct xenvif_rx_cb {
+ int meta_slots_used;
+ bool full_coalesce;
+};
+
+#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
+
/*
* Set up the grant operations for this fragment. If it's a flipping
* interface, we also set up the unmap request from here.
@@ -261,7 +277,10 @@ static void xenvif_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,
+ XENVIF_RX_CB(skb)->full_coalesce)) {
/*
* Netfront requires there to be some data in the head
* buffer.
@@ -541,12 +560,6 @@ static void xenvif_add_frag_responses(struct xenvif *vif,
int status,
}
}
-struct xenvif_rx_cb {
- int meta_slots_used;
-};
-
-#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
-
void xenvif_kick_thread(struct xenvif *vif)
{
wake_up(&vif->wq);
@@ -602,10 +615,14 @@ static void xenvif_rx_action(struct xenvif *vif)
/* To avoid the estimate becoming too pessimal for some
* frontends that limit posted rx requests, cap the estimate
- * at MAX_SKB_FRAGS.
+ * at MAX_SKB_FRAGS. In this case netback will fully coalesce
+ * the skb into the provided slots.
*/
- if (max_slots_needed > MAX_SKB_FRAGS)
+ if (max_slots_needed > MAX_SKB_FRAGS) {
max_slots_needed = MAX_SKB_FRAGS;
+ XENVIF_RX_CB(skb)->full_coalesce = true;
+ } else
+ XENVIF_RX_CB(skb)->full_coalesce = false;
/* We may need one more slot for GSO metadata */
if (skb_is_gso(skb) &&
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