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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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)?

Plus I think the increment of pages would need to be pulled out
of the if() body.

> +     }
> +
> +     return pages;
> +}
> +
>  static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>       unsigned short id;
> @@ -517,12 +557,13 @@ 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 frags;
>       unsigned int offset = offset_in_page(data);
>       unsigned int len = skb_headlen(skb);
>       unsigned long flags;
>  
> -     frags += DIV_ROUND_UP(offset + len, PAGE_SIZE);
> +     frags = xennet_count_skb_frag_pages(skb) +
> +             DIV_ROUND_UP(offset + len, PAGE_SIZE);
>       if (unlikely(frags > MAX_SKB_FRAGS + 1)) {

This condition would now need adjustment, though (because
"frags" is no longer what its name says).

Jan

>               printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n",
>                      frags);




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