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Date: Wed, 10 Oct 2012 11:13:04 +0100
From: Ian Campbell <Ian.Campbell@...rix.com>
To: Eric Dumazet <eric.dumazet@...il.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Sander Eikelenboom <linux@...elenboom.it>,
"Konrad Rzeszutek Wilk" <konrad@...nel.org>,
xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] compound skb frag pages appearing in start_xmit
On Tue, 2012-10-09 at 15:40 +0100, Ian Campbell wrote:
> On Tue, 2012-10-09 at 15:27 +0100, Eric Dumazet wrote:
> > On Tue, 2012-10-09 at 15:17 +0100, Ian Campbell wrote:
> >
> > > Does the higher order pages effectively reduce the number of frags which
> > > are in use? e.g if MAX_SKB_FRAGS is 16, then for order-0 pages you could
> > > have 64K worth of frag data.
> > >
> > > If we switch to order-3 pages everywhere then can the skb contain 512K
> > > of data, or does the effective maximum number of frags in an skb reduce
> > > to 2?
> >
> > effective number of frags reduce to 2 or 3
> >
> > (We still limit GSO packets to ~63536 bytes)
>
> Great! Then I think the fix is more/less trivial...
The following seems to work for me.
I haven't tackled netfront yet.
8<--------------------------------------------------------------
>From 551e42e3dd203f2eb97cb082985013bb33b8f020 Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@...rix.com>
Date: Tue, 9 Oct 2012 15:51:20 +0100
Subject: [PATCH] xen: netback: handle compound page fragments on transmit.
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 netbk_gop_frag_copy and xen_netbk_count_skb_slots by
iterating over the frames which make up the page.
Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Konrad Rzeszutek Wilk <konrad@...nel.org>
Cc: Sander Eikelenboom <linux@...elenboom.it>
---
drivers/net/xen-netback/netback.c | 40 ++++++++++++++++++++++++++++++++----
1 files changed, 35 insertions(+), 5 deletions(-)
diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 4ebfcf3..d747e30 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -335,21 +335,35 @@ unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
+ unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
unsigned long bytes;
+
+ offset &= ~PAGE_MASK;
+
while (size > 0) {
+ BUG_ON(offset >= PAGE_SIZE);
BUG_ON(copy_off > MAX_BUFFER_OFFSET);
- if (start_new_rx_buffer(copy_off, size, 0)) {
+ bytes = PAGE_SIZE - offset;
+
+ if (bytes > size)
+ bytes = size;
+
+ if (start_new_rx_buffer(copy_off, bytes, 0)) {
count++;
copy_off = 0;
}
- bytes = size;
if (copy_off + bytes > MAX_BUFFER_OFFSET)
bytes = MAX_BUFFER_OFFSET - copy_off;
copy_off += bytes;
+
+ offset += bytes;
size -= bytes;
+
+ if (offset == PAGE_SIZE)
+ offset = 0;
}
}
return count;
@@ -403,14 +417,24 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
unsigned long bytes;
/* Data must not cross a page boundary. */
- BUG_ON(size + offset > PAGE_SIZE);
+ BUG_ON(size + offset > PAGE_SIZE<<compound_order(page));
meta = npo->meta + npo->meta_prod - 1;
+ /* Skip unused frames from start of page */
+ page += offset >> PAGE_SHIFT;
+ offset &= ~PAGE_MASK;
+
while (size > 0) {
+ BUG_ON(offset >= PAGE_SIZE);
BUG_ON(npo->copy_off > MAX_BUFFER_OFFSET);
- if (start_new_rx_buffer(npo->copy_off, size, *head)) {
+ bytes = PAGE_SIZE - offset;
+
+ if (bytes > size)
+ bytes = size;
+
+ if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
/*
* Netfront requires there to be some data in the head
* buffer.
@@ -420,7 +444,6 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
meta = get_next_rx_buffer(vif, npo);
}
- bytes = size;
if (npo->copy_off + bytes > MAX_BUFFER_OFFSET)
bytes = MAX_BUFFER_OFFSET - npo->copy_off;
@@ -453,6 +476,13 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
offset += bytes;
size -= bytes;
+ /* Next frame */
+ if (offset == PAGE_SIZE) {
+ BUG_ON(!PageCompound(page));
+ page++;
+ offset = 0;
+ }
+
/* Leave a gap for the GSO descriptor. */
if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
vif->rx.req_cons++;
--
1.7.2.5
--
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