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Date: Fri, 16 May 2014 17:29:05 +0100
From: Zoltan Kiss <zoltan.kiss@...rix.com>
To: Wei Liu <wei.liu2@...rix.com>,
Eric Dumazet <eric.dumazet@...il.com>
CC: <netdev@...r.kernel.org>, <xen-devel@...ts.xen.org>,
David Vrabel <david.vrabel@...rix.com>,
Konrad Wilk <konrad.wilk@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Stefan Bader <stefan.bader@...onical.com>
Subject: Re: [PATCH net-next] xen-netfront: try linearizing SKB if it occupies
too many slots
On 16/05/14 16:34, Wei Liu wrote:
> On Fri, May 16, 2014 at 08:22:19AM -0700, Eric Dumazet wrote:
>> On Fri, 2014-05-16 at 15:36 +0100, Wei Liu wrote:
>>> On Fri, May 16, 2014 at 07:21:08AM -0700, Eric Dumazet wrote:
>>>> On Fri, 2014-05-16 at 14:11 +0100, Wei Liu wrote:
>>>>
>>>>> It's not that common to trigger this, I only saw a few reports. In fact
>>>>> Stefan's report is the first one that comes with a method to reproduce
>>>>> it.
>>>>>
>>>>> I tested with redis-benchmark on a guest with 256MB RAM and only saw a
>>>>> few "failed to linearize", never saw a single one with 1GB guest.
>>>>
>>>> Well, I am just saying. This is asking order-5 allocations, and yes,
>>>> this is going to fail after few days of uptime, no matter what you try.
>>>>
>>>
>>> Hmm... I see what you mean -- memory fragmentation leads to allocation
>>> failure. Thanks.
>>
>> In the mean time, have you tried to lower gso_max_size ?
>>
>> Setting it witk netif_set_gso_max_size() to something like 56000 might
>> avoid the problem.
>>
>> (Not sure if it is applicable in your case)
>>
>
> It works, at least in this Redis testcase. Could you explain a bit where
> this 56000 magic number comes from? :-)
>
> Presumably I can derive it from some constant in core network code?
I guess it just makes more unlikely to have packets with problematic layout. But the following packet would still fail:
linear buffer : 80 bytes, on 2 pages
17 frags, 80 bytes each, each spanning over page boundary.
I just had an idea: a modified version of xenvif_handle_frag_list function from netback would be useful for us here. It recreates the frags array on fully utilized 4k pages. Plus we can use pskb_expand_head to reduce the page number on the linear buffer (although it might not work, see my comment in the patch)
The worst case linear buffer then spans N+1 pages, and has N*PAGE_SIZE+1 bytes. Then the frags after this coalescing should have 16*PAGE_SIZE - (N*PAGE_SIZE+2) bytes at most, which is 16-N pages. Altogether that's 16+1 page, which should definitely fit!
This is what I mean:
8<--------------
diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 158b5e6..b1133d6 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -544,6 +544,73 @@ static int xennet_count_skb_frag_slots(struct sk_buff *skb)
return pages;
}
+int xenvif_reduce_pages(struct sk_buff *skb, int target)
+{
+ unsigned int offset = skb_headlen(skb);
+ skb_frag_t frags[MAX_SKB_FRAGS];
+ int newfrags, oldfrags;
+ unsigned int pages, optimal;
+
+ BUG_ON(!target);
+
+ pages = DIV_ROUND_UP(offset_in_page(skb->data) + skb_headlen(skb), PAGE_SIZE);
+ optimal = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
+ if (pages - optimal) {
+ int err;
+/* FIXME: we should check if pskb_expand_head really allocates on page boundary,
+ * otherwise we can still have suboptimal page layout */
+ if (unlikely(err = pskb_expand_head(skb, 0, 0, GFP_ATOMIC)))
+ return err;
+ target -= pages - optimal;
+ if (!target)
+ return 0;
+ }
+
+ /* Subtract frags size, we will correct it later */
+ skb->truesize -= skb->data_len;
+
+ /* Create a brand new frags array and coalesce there */
+ for (newfrags = 0; offset < skb->len; newfrags++) {
+ struct page *page;
+ unsigned int len;
+
+ BUG_ON(newfrags >= MAX_SKB_FRAGS);
+ page = alloc_page(GFP_ATOMIC);
+ if (!page) {
+ int j;
+ skb->truesize += skb->data_len;
+ for (j = 0; j < newfrags; j++)
+ put_page(frags[j].page.p);
+ return -ENOMEM;
+ }
+
+ if (offset + PAGE_SIZE < skb->len)
+ len = PAGE_SIZE;
+ else
+ len = skb->len - offset;
+ if (skb_copy_bits(skb, offset, page_address(page), len))
+ BUG();
+
+ offset += len;
+ frags[newfrags].page.p = page;
+ frags[newfrags].page_offset = 0;
+ skb_frag_size_set(&frags[newfrags], len);
+ }
+
+ /* Drop the original buffers */
+ for (oldfrags = 0; oldfrags < skb_shinfo(skb)->nr_frags; oldfrags++)
+ skb_frag_unref(skb, oldfrags);
+
+ /* Swap the new frags array with the old one */
+ memcpy(skb_shinfo(skb)->frags,
+ frags,
+ newfrags * sizeof(skb_frag_t));
+ skb_shinfo(skb)->nr_frags = newfrags;
+ /* Correct truesize */
+ skb->truesize += newfrags * PAGE_SIZE;
+ return 0;
+}
+
static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
{
unsigned short id;
@@ -573,11 +640,21 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
xennet_count_skb_frag_slots(skb);
if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
- net_alert_ratelimited(
- "xennet: skb rides the rocket: %d slots\n", slots);
- goto drop;
+ if (unlikely(xenvif_reduce_pages(skb, slots - MAX_SKB_FRAGS - 1))) {
+ net_alert_ratelimited(
+ "xennet: couldn't reduce slot number from %d\n", slots);
+ goto drop;
+ }
+ slots = DIV_ROUND_UP(offset_in_page(data) + len, PAGE_SIZE) +
+ xennet_count_skb_frag_slots(skb);
+ if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
+ net_alert_ratelimited(
+ "xennet: slot reduction doesn't work, slots: %d\n", slots);
+ goto drop;
+ }
}
+
spin_lock_irqsave(&np->tx_lock, flags);
if (unlikely(!netif_carrier_ok(dev) ||
--
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