netdev - Re: [PATCH] netvm: check for page == NULL when propogating the skb->pfmemalloc flag

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Message-ID: <1345631207.6821.140.camel@zakaz.uk.xensource.com>
Date:	Wed, 22 Aug 2012 11:26:47 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	David Miller <davem@...emloft.net>
CC:	"mgorman@...e.de" <mgorman@...e.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"konrad@...nok.org" <konrad@...nok.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] netvm: check for page == NULL when propogating the
 skb->pfmemalloc flag

On Wed, 2012-08-08 at 23:50 +0100, David Miller wrote:
> Just use something like a call to __pskb_pull_tail(skb, len) and all
> that other crap around that area can simply be deleted.

I think you mean something like this, which works for me, although I've
only lightly tested it.

Ian.

8<----------------------------------------

>From 9e47e3e87a33b45974448649a97859a479183041 Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@...rix.com>
Date: Wed, 22 Aug 2012 10:15:29 +0100
Subject: [PATCH] xen-netfront: use __pskb_pull_tail to ensure linear area is big enough on RX

I'm slightly concerned by the "only in exceptional circumstances"
comment on __pskb_pull_tail but the structure of an skb just created
by netfront shouldn't hit any of the especially slow cases.

This approach still does slightly more work than the old way, since if
we pull up the entire first frag we now have to shuffle everything
down where before we just received into the right place in the first
place.

Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: Jeremy Fitzhardinge <jeremy@...p.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: xen-devel@...ts.xensource.com
Cc: netdev@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
---
 drivers/net/xen-netfront.c |   39 ++++++++++-----------------------------
 1 files changed, 10 insertions(+), 29 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 3089990..650f79a 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -57,8 +57,7 @@
 static const struct ethtool_ops xennet_ethtool_ops;
 
 struct netfront_cb {
-	struct page *page;
-	unsigned offset;
+	int pull_to;
 };
 
 #define NETFRONT_SKB_CB(skb)	((struct netfront_cb *)((skb)->cb))
@@ -867,15 +866,9 @@ static int handle_incoming_queue(struct net_device *dev,
 	struct sk_buff *skb;
 
 	while ((skb = __skb_dequeue(rxq)) != NULL) {
-		struct page *page = NETFRONT_SKB_CB(skb)->page;
-		void *vaddr = page_address(page);
-		unsigned offset = NETFRONT_SKB_CB(skb)->offset;
-
-		memcpy(skb->data, vaddr + offset,
-		       skb_headlen(skb));
+		int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
 
-		if (page != skb_frag_page(&skb_shinfo(skb)->frags[0]))
-			__free_page(page);
+		__pskb_pull_tail(skb, pull_to - skb_headlen(skb));
 
 		/* Ethernet work: Delayed to here as it peeks the header. */
 		skb->protocol = eth_type_trans(skb, dev);
@@ -913,7 +906,6 @@ static int xennet_poll(struct napi_struct *napi, int budget)
 	struct sk_buff_head errq;
 	struct sk_buff_head tmpq;
 	unsigned long flags;
-	unsigned int len;
 	int err;
 
 	spin_lock(&np->rx_lock);
@@ -955,24 +947,13 @@ err:
 			}
 		}
 
-		NETFRONT_SKB_CB(skb)->page =
-			skb_frag_page(&skb_shinfo(skb)->frags[0]);
-		NETFRONT_SKB_CB(skb)->offset = rx->offset;
-
-		len = rx->status;
-		if (len > RX_COPY_THRESHOLD)
-			len = RX_COPY_THRESHOLD;
-		skb_put(skb, len);
+		NETFRONT_SKB_CB(skb)->pull_to = rx->status;
+		if (NETFRONT_SKB_CB(skb)->pull_to > RX_COPY_THRESHOLD)
+			NETFRONT_SKB_CB(skb)->pull_to = RX_COPY_THRESHOLD;
 
-		if (rx->status > len) {
-			skb_shinfo(skb)->frags[0].page_offset =
-				rx->offset + len;
-			skb_frag_size_set(&skb_shinfo(skb)->frags[0], rx->status - len);
-			skb->data_len = rx->status - len;
-		} else {
-			__skb_fill_page_desc(skb, 0, NULL, 0, 0);
-			skb_shinfo(skb)->nr_frags = 0;
-		}
+		skb_shinfo(skb)->frags[0].page_offset = rx->offset;
+		skb_frag_size_set(&skb_shinfo(skb)->frags[0], rx->status);
+		skb->data_len = rx->status;
 
 		i = xennet_fill_frags(np, skb, &tmpq);
 
@@ -999,7 +980,7 @@ err:
 		 * receive throughout using the standard receive
 		 * buffer size was cut by 25%(!!!).
 		 */
-		skb->truesize += skb->data_len - (RX_COPY_THRESHOLD - len);
+		skb->truesize += skb->data_len - RX_COPY_THRESHOLD;
 		skb->len += skb->data_len;
 
 		if (rx->flags & XEN_NETRXF_csum_blank)
-- 
1.7.2.5



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