Re: [PATCH net-next] xen-netfront: try linearizing SKB if it occupies too many slots
From: Zoltan Kiss <hidden>
Date: 2014-05-16 16:29:09
Subsystem:
networking drivers, the rest, xen hypervisor interface · Maintainers:
Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds, Juergen Gross, Stefano Stabellini
On 16/05/14 16:34, Wei Liu wrote:
On Fri, May 16, 2014 at 08:22:19AM -0700, Eric Dumazet wrote:quoted
On Fri, 2014-05-16 at 15:36 +0100, Wei Liu wrote:quoted
On Fri, May 16, 2014 at 07:21:08AM -0700, Eric Dumazet wrote:quoted
On Fri, 2014-05-16 at 14:11 +0100, Wei Liu wrote:quoted
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) ||