Re: [Xen-devel] [PATCH] xen/netfront: handle compound page fragments on transmit
flat view
From: Ian Campbell <hidden>
Date: 2012-11-20 13:35:19
Subsystem:
networking drivers, the rest, xen hypervisor interface · Maintainers:
Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds, Juergen Gross, Stefano Stabellini
On Tue, 2012-11-20 at 12:28 +0000, Jan Beulich wrote:
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On 20.11.12 at 12:40, Ian Campbell [off-list ref] 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, but how about this incremental patch:
diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index a12b99a..06d0a84 100644
--- 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; + } + } + } + + 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)) { printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n", frags);