Subject: Re: Data corruption issue with splice() on 2.6.27.10

From: Jarek Poplawski <[email-masked]>
Date: Wed, 7 Jan 2009 09:39:15 +0000

On Tue, Jan 06, 2009 at 04:57:15PM +0100, Willy Tarreau wrote:
> On Tue, Jan 06, 2009 at 10:01:13AM +0000, Jarek Poplawski wrote:
> > On Tue, Jan 06, 2009 at 10:41:38AM +0100, Willy Tarreau wrote:
> > ...
> > > > Great story! Alas I don't understand this fully either, but it seems
> > > > Changli Gao was concerned with sendpage sending this "as pages", so
> > > > when NETIF_F_SG flag is available. Did you try this without SG btw?
> > > 
> > > No I did not. I can try, it's not too hard. It would in part defeat the
> > > purpose of the mechanism (especially at 10 Gbps) but at least it will
> > > help narrow the problem down.
> > 
> > Yes, I meant it only as a proof of concept. BTW, delaying TCP acks a
> > bit for these sendpages should then make it more reproducible, I guess.
> 
> OK here is an update. It does not change anything to turn off any acceleration
> feature on the interface (tg3) :
> 
> root@wtap:~# ethtool -k eth0
> Offload parameters for eth0:
> rx-checksumming: off
> tx-checksumming: off
> scatter-gather: off
> tcp segmentation offload: off
> 
> It still forwards corrupted data like mad. I noticed that the corruption rate
> is 10-100 times higher when forwarding from eth0 to eth0 than from eth0 to lo.
> 
> Maybe this can help find the culprit ?

I hope it will, but I still don't get it. Anyway, here is an untested
patch, which I guess partly tries Changli Gao's recommendation to give
real pages to splice/pipe (but here it's always - not for sendpage
only).

BTW, I added Changli to Cc - great review!

Thanks,
Jarek P.

---

 net/core/skbuff.c |   41 +++++++++++++++++++++++++++--------------
 1 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5110b35..4c080cd 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -73,17 +73,13 @@ static struct kmem_cache *skbuff_fclone_cache __read_mostly;
 static void sock_pipe_buf_release(struct pipe_inode_info *pipe,
 				  struct pipe_buffer *buf)
 {
-	struct sk_buff *skb = (struct sk_buff *) buf->private;
-
-	kfree_skb(skb);
+	put_page(buf->page);
 }
 
 static void sock_pipe_buf_get(struct pipe_inode_info *pipe,
 				struct pipe_buffer *buf)
 {
-	struct sk_buff *skb = (struct sk_buff *) buf->private;
-
-	skb_get(skb);
+	get_page(buf->page);
 }
 
 static int sock_pipe_buf_steal(struct pipe_inode_info *pipe,
@@ -1334,9 +1330,19 @@ fault:
  */
 static void sock_spd_release(struct splice_pipe_desc *spd, unsigned int i)
 {
-	struct sk_buff *skb = (struct sk_buff *) spd->partial[i].private;
+	put_page(spd->pages[i]);
+}
 
-	kfree_skb(skb);
+static inline struct page *linear_to_page(struct page *page, unsigned int len,
+					  unsigned int offset)
+{
+	struct page *p = alloc_pages(GFP_KERNEL, 0);
+
+	if (!p)
+		return NULL;
+	memcpy(p + offset, page + offset, len);
+
+	return p;
 }
 
 /*
@@ -1344,16 +1350,23 @@ static void sock_spd_release(struct splice_pipe_desc *spd, unsigned int i)
  */
 static inline int spd_fill_page(struct splice_pipe_desc *spd, struct page *page,
 				unsigned int len, unsigned int offset,
-				struct sk_buff *skb)
+				struct sk_buff *skb, int linear)
 {
 	if (unlikely(spd->nr_pages == PIPE_BUFFERS))
 		return 1;
 
+	if (linear) {
+		page = linear_to_page(page, len, offset);
+		if (!page)
+			return 1;
+	}
+
 	spd->pages[spd->nr_pages] = page;
 	spd->partial[spd->nr_pages].len = len;
 	spd->partial[spd->nr_pages].offset = offset;
-	spd->partial[spd->nr_pages].private = (unsigned long) skb_get(skb);
 	spd->nr_pages++;
+	get_page(page);
+
 	return 0;
 }
 
@@ -1369,7 +1382,7 @@ static inline void __segment_seek(struct page **page, unsigned int *poff,
 static inline int __splice_segment(struct page *page, unsigned int poff,
 				   unsigned int plen, unsigned int *off,
 				   unsigned int *len, struct sk_buff *skb,
-				   struct splice_pipe_desc *spd)
+				   struct splice_pipe_desc *spd, int linear)
 {
 	if (!*len)
 		return 1;
@@ -1392,7 +1405,7 @@ static inline int __splice_segment(struct page *page, unsigned int poff,
 		/* the linear region may spread across several pages  */
 		flen = min_t(unsigned int, flen, PAGE_SIZE - poff);
 
-		if (spd_fill_page(spd, page, flen, poff, skb))
+		if (spd_fill_page(spd, page, flen, poff, skb, linear))
 			return 1;
 
 		__segment_seek(&page, &poff, &plen, flen);
@@ -1419,7 +1432,7 @@ static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
 	if (__splice_segment(virt_to_page(skb->data),
 			     (unsigned long) skb->data & (PAGE_SIZE - 1),
 			     skb_headlen(skb),
-			     offset, len, skb, spd))
+			     offset, len, skb, spd, 1))
 		return 1;
 
 	/*
@@ -1429,7 +1442,7 @@ static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
 		const skb_frag_t *f = &skb_shinfo(skb)->frags[seg];
 
 		if (__splice_segment(f->page, f->page_offset, f->size,
-				     offset, len, skb, spd))
+				     offset, len, skb, spd, 0))
 			return 1;
 	}