[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen/netfront: handle compound page fragments on transmit
>>> 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); _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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