On Mon, Dec 16, 2013 at 04:10:42PM +0000, Zoltan Kiss wrote:

On 13/12/13 15:43, Wei Liu wrote:

quoted

On Thu, Dec 12, 2013 at 11:48:14PM +0000, Zoltan Kiss wrote:

quoted

Xen network protocol had implicit dependency on MAX_SKB_FRAGS. Netback has to
handle guests sending up to XEN_NETBK_LEGACY_SLOTS_MAX slots. To achieve that:
- create a new skb
- map the leftover slots to its frags (no linear buffer here!)
- chain it to the previous through skb_shinfo(skb)->frag_list
- map them
- copy the whole stuff into a brand new skb and send it to the stack
- unmap the 2 old skb's pages

Do you see performance regression with this approach?

Well, it was pretty hard to reproduce that behaviour even with NFS.
I don't think it happens often enough that it causes a noticable
performance regression. Anyway, it would be just as slow as the
current grant copy with coalescing, maybe a bit slower due to the
unmapping. But at least we use a core network function to do the
coalescing.
Or, if you mean the generic performance, if this problem doesn't
appear, then no, I don't see performance regression.

OK, thanks for comfirming.

quoted

quoted

Signed-off-by: Zoltan Kiss <redacted>

---
 drivers/net/xen-netback/netback.c |   99 +++++++++++++++++++++++++++++++++++--
 1 file changed, 94 insertions(+), 5 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index e26cdda..f6ed1c8 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c

@@ -906,11 +906,15 @@ static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,
 	u16 pending_idx = *((u16 *)skb->data);
 	int start;
 	pending_ring_idx_t index;
-	unsigned int nr_slots;
+	unsigned int nr_slots, frag_overflow = 0;

 	/* At this point shinfo->nr_frags is in fact the number of
 	 * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
 	 */
+	if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+		frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;
+		shinfo->nr_frags = MAX_SKB_FRAGS;
+	}
 	nr_slots = shinfo->nr_frags;

It is also probably better to check whether shinfo->nr_frags is too
large which makes frag_overflow > MAX_SKB_FRAGS. I know skb should be
already be valid at this point but it wouldn't hurt to be more careful.

Ok, I've added this:
	/* At this point shinfo->nr_frags is in fact the number of
	 * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
	 */
+	if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+		if (shinfo->nr_frags > XEN_NETBK_LEGACY_SLOTS_MAX) return NULL;
+		frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;

What I suggested is

   BUG_ON(frag_overflow > MAX_SKB_FRAGS)

Wei.