Eric Lee / smarc-ti-linux-kernel

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Commit 97a6d1bb2b658ac85ed88205ccd1ab809899884d

Authored by Zoltan Kiss
Committed by David S. Miller
1 parent 407066f8f3
Exists in ti-lsk-linux-4.1.y and in 10 other branches dlt-processor-sdk-linux-03.00.00.04, processor-sdk-linux-02.00.01, smarc-ti-lsk-linux-4.1.y, smarct3x-processor-sdk-04.01.00.06, smarct3x-processor-sdk-linux-02.00.01, smarct3x-processor-sdk-linux-03.00.00.04, smarct4x-800-processor-sdk-linux-02.00.01, smarct4x-processor-sdk-04.01.00.06, smarct4x-processor-sdk-linux-02.00.01, smarct4x-processor-sdk-linux-03.00.00.04

xen-netfront: Fix handling packets on compound pages with skb_linearize 

There is a long known problem with the netfront/netback interface: if the guest
tries to send a packet which constitues more than MAX_SKB_FRAGS + 1 ring slots,
it gets dropped. The reason is that netback maps these slots to a frag in the
frags array, which is limited by size. Having so many slots can occur since
compound pages were introduced, as the ring protocol slice them up into
individual (non-compound) page aligned slots. The theoretical worst case
scenario looks like this (note, skbs are limited to 64 Kb here):
linear buffer: at most PAGE_SIZE - 17 * 2 bytes, overlapping page boundary,
using 2 slots
first 15 frags: 1 + PAGE_SIZE + 1 bytes long, first and last bytes are at the
end and the beginning of a page, therefore they use 3 * 15 = 45 slots
last 2 frags: 1 + 1 bytes, overlapping page boundary, 2 * 2 = 4 slots
Although I don't think this 51 slots skb can really happen, we need a solution
which can deal with every scenario. In real life there is only a few slots
overdue, but usually it causes the TCP stream to be blocked, as the retry will
most likely have the same buffer layout.
This patch solves this problem by linearizing the packet. This is not the
fastest way, and it can fail much easier as it tries to allocate a big linear
area for the whole packet, but probably easier by an order of magnitude than
anything else. Probably this code path is not touched very frequently anyway.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Paul Durrant <paul.durrant@citrix.com>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Signed-off-by: David S. Miller <davem@davemloft.net>

Showing 1 changed file with 4 additions and 3 deletions Side-by-side Diff

drivers/net/xen-netfront.c
Diff comments   View file @ 97a6d1b
... ... @@ -628,9 +628,10 @@
628 628 slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
629 629 xennet_count_skb_frag_slots(skb);
630 630 if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
631   - net_alert_ratelimited(
632   - "xennet: skb rides the rocket: %d slots\n", slots);
633   - goto drop;
  631 + net_dbg_ratelimited("xennet: skb rides the rocket: %d slots, %d bytes\n",
  632 + slots, skb->len);
  633 + if (skb_linearize(skb))
  634 + goto drop;
634 635 }
635 636  
636 637 spin_lock_irqsave(&queue->tx_lock, flags);