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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.11.12 at 15:14, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Tue, 2012-11-20 at 13:51 +0000, Jan Beulich wrote:
>> >>> On 20.11.12 at 14:35, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> > On Tue, 2012-11-20 at 12:28 +0000, Jan Beulich wrote:
>> >> >>> On 20.11.12 at 12:40, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
>> >> > An SKB paged fragment can consist of a compound page with order > 0.
>> >> > However the netchannel protocol deals only in PAGE_SIZE frames.
>> >> > 
>> >> > Handle this in xennet_make_frags by iterating over the frames which
>> >> > make up the page.
>> >> > 
>> >> > This is the netfront equivalent to 6a8ed462f16b for netback.
>> >> 
>> >> Wouldn't you need to be at least a little more conservative here
>> >> with respect to resource use: I realize that get_id_from_freelist()
>> >> return values were never checked, and failure of
>> >> gnttab_claim_grant_reference() was always dealt with via
>> >> BUG_ON(), but considering that netfront_tx_slot_available()
>> >> doesn't account for compound page fragments, I think this (lack
>> >> of) error handling needs improvement in the course of the
>> >> change here (regardless of - I think - someone having said that
>> >> usually the sum of all pages referenced from an skb's fragments
>> >> would not exceed MAX_SKB_FRAGS - "usually" just isn't enough
>> >> imo).
>> > 
>> > I think it is more than "usually", it is derived from the number of
>> > pages needed to contain 64K of data which is the maximum size of the
>> > data associated with an skb (AIUI).
>> > 
>> > Unwinding from failure in xennet_make_frags looks pretty tricky,
>> 
>> Yes, I agree.
>> 
>> > but how about this incremental patch:
>> 
>> Looks good, but can probably be simplified quite a bit:
>> 
>> > --- a/drivers/net/xen-netfront.c
>> > +++ b/drivers/net/xen-netfront.c
>> > @@ -505,6 +505,46 @@ static void xennet_make_frags(struct sk_buff *skb, 
> struct net_device *dev,
>> >    np->tx.req_prod_pvt = prod;
>> >  }
>> >  
>> > +/*
>> > + * Count how many ring slots are required to send the frags of this
>> > + * skb. Each frag might be a compound page.
>> > + */
>> > +static int xennet_count_skb_frag_pages(struct sk_buff *skb)
>> > +{
>> > +  int i, frags = skb_shinfo(skb)->nr_frags;
>> > +  int pages = 0;
>> > +
>> > +  for (i = 0; i < frags; i++) {
>> > +          skb_frag_t *frag = skb_shinfo(skb)->frags + i;
>> > +          unsigned long size = skb_frag_size(frag);
>> > +          unsigned long offset = frag->page_offset;
>> > +
>> > +          /* Skip unused frames from start of page */
>> > +          offset &= ~PAGE_MASK;
>> > +
>> > +          while (size > 0) {
>> > +                  unsigned long bytes;
>> > +
>> > +                  BUG_ON(offset >= PAGE_SIZE);
>> > +
>> > +                  bytes = PAGE_SIZE - offset;
>> > +                  if (bytes > size)
>> > +                          bytes = size;
>> > +
>> > +                  offset += bytes;
>> > +                  size -= bytes;
>> > +
>> > +                  /* Next frame */
>> > +                  if (offset == PAGE_SIZE && size) {
>> > +                          pages++;
>> > +                          offset = 0;
>> > +                  }
>> > +          }
>> 
>> Isn't the whole loop equivalent to 
>> 
>>              pages = PFN_UP(offset + size);
>> 
>> (at least as long as size is not zero)?
> 
> Er, yes. Wood for the trees etc...
> 
> I think using PFN_UP overcounts a bit since the data needed start in the
> first frame of a compound frame, but if you keep the 
>         /* Skip unused frames from start of page */
>         offset &= ~PAGE_MASK;
>         
> I think that does the right thing

Right, that's what I said (I only wanted the loop to be replaced, not
what was prior to it).

> @@ -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)?

Jan

> +             printk(KERN_ALERT "xennet: skb rides the rocket: %d slots\n",
> +                    slots);
>               dump_stack();
>               goto drop;
>       }


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