netdev - Re: [Xen-devel] [PATCH] xen/netfront: handle compound page fragments on transmit

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Date:	Tue, 20 Nov 2012 15:44:50 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ian Campbell" <Ian.Campbell@...rix.com>
Cc:	"Stefan Bader" <stefan.bader@...onical.com>,
	"Sander Eikelenboom" <linux@...elenboom.it>,
	"Eric Dumazet" <edumazet@...gle.com>,
	"KonradRzeszutekWilk" <konrad@...nel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"ANNIE LI" <annie.li@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/netfront: handle compound page
 fragments on transmit

>>> On 20.11.12 at 16:06, Ian Campbell <Ian.Campbell@...rix.com> wrote:
> On Tue, 2012-11-20 at 14:32 +0000, Jan Beulich wrote:
>> > @@ -517,15 +540,16 @@ static int xennet_start_xmit(struct sk_buff *skb, 
>> > struct net_device *dev)
>> >  	grant_ref_t ref;
>> >  	unsigned long mfn;
>> >  	int notify;
>> > -	int frags = skb_shinfo(skb)->nr_frags;
>> > +	int slots;
>> >  	unsigned int offset = offset_in_page(data);
>> >  	unsigned int len = skb_headlen(skb);
>> >  	unsigned long flags;
>> >  
>> > -	frags += DIV_ROUND_UP(offset + len, PAGE_SIZE);
>> > -	if (unlikely(frags > MAX_SKB_FRAGS + 1)) {
>> > -		printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n",
>> > -		       frags);
>> > +	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
>> > +		xennet_count_skb_frag_slots(skb);
>> > +	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
>> 
>> But still - isn't this wrong now (i.e. can't it now validly exceed the
>> boundary checked for)?
> 
> In practice no because of the property that the number of pages backing
> the frags is <= MAX_SKB_FRAGS even if you are using compound pages as
> the frags.

So are you saying that there is something in the system
preventing up to MAX_SKB_FRAGS * SKB_FRAG_PAGE_ORDER
(or NETDEV_FRAG_PAGE_MAX_ORDER) skb-s to be created? I
didn't find any. I do notice that __netdev_alloc_frag() currently
never gets called with a size larger than PAGE_SIZE, but
considering that the function just recently got made capable of
that, I'm sure respective users will show up rather sooner than
later.

Jan

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