Re: [PATCH net-next] xen-netfront: try linearizing SKB if it occupies too many slots

[Wei Liu <wei.liu2@citrix.com>]

On Fri, May 16, 2014 at 05:29:05PM +0100, Zoltan Kiss wrote:
> 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.
> 

Presumably max GSO size affects packet layout, that's what I was trying
to figure out.

> 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)))

I'm a bit lost. What do you expect from the call to pskb_expand_head?

I'm sorry I cannot see immediate result from the comment of
pskb_expand_head. If you call with nhead and ntail equal to 0 it creates
identical copy, but I don't see guarantee on page alignment. Did I miss
something?

> +			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);

And the ammount of memory allocation is a bit overkill I think (though
it's still better than the order-5 allocation in skb_linearize). Can you
not just memmove all paged data to first few frags and release other
frags?

Anyway, this method might still work, just a bit overkill IMHO.

Wei.

> +		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) ||