Subject: Re: kernel 3.2.27 on arm: WARNING: at mm/page_alloc.c:2109
 __alloc_pages_nodemask+0x1d4/0x68c()
From: Eric Dumazet <eric.dumazet@gmail.com>
To: mbizon@freebox.fr
Cc: David Madore <david+ml@madore.org>, Francois Romieu <romieu@fr.zoreil.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Hugh Dickins <hughd@google.com>
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Date: Fri, 05 Oct 2012 14:37:47 +0200
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On Fri, 2012-10-05 at 14:22 +0200, Eric Dumazet wrote:
> On Fri, 2012-10-05 at 12:49 +0200, Maxime Bizon wrote:
> > On Fri, 2012-10-05 at 09:41 +0200, Eric Dumazet wrote:
> > 
> > > By the way, the commit you pointed has no effect on the reallocation
> > > performed by pskb_expand_head() :
> > 
> > The commit has a side effect, because the problem appeared after it was
> > merged (and goes away if I revert it)
> > 
> > > int size = nhead + skb_end_offset(skb) + ntail;
> > > 
> > > So pskb_expand_head() always assumed the current head is fully used, and
> > > because we have some kmalloc-power-of-two contraints, each time
> > > pskb_expand_head() is called with a non zero (nhead + ntail) we double
> > > the skb->head ksize.
> > 
> > That is true, but only after the commit I mentioned.
> > 
> > Before that commit, we indeed reallocate skb->head to twice the size,
> > but skb->end is *not* positioned at the end of newly allocated data. So
> > on the next pskb_expand_head(), if head and tail are not big values, the
> > kmalloc() will be of the same size.
> > 
> > 
> > The commit adds this after allocation:
> > 
> > size = SKB_WITH_OVERHEAD(ksize(data))
> > [...]
> > skb->end      = skb->head + size;
> > 
> > so on the next pskb_expand_head, we are going to allocate twice the size
> > for sure.
> 
> Yes, but the idea of the patch was to _avoid_ next pskb_expand_head()
> calls...
> 
> Its defeated because you have a too small NET_SKB_PAD, and skb_recycle()
> inability to properly detect ans skb is oversized.
> 
> > 
> > > So why are we using skb_end_offset(skb) here is the question.
> > > 
> > > I guess it could be (skb_tail_pointer(skb) - skb->head) on some uses.
> > 
> > I think your patch is wrong, ntail is not the new tailroom size, it's
> > what missing to the current tailroom size, by adding ntail + nhead +
> > tail_offset we are removing previous tailroom.
> > 
> 
> 
> 
> > We cannot shrink the skb that way here I guess, a caller may check
> > needed headroom & tailroom, calls with nhead=1/ntail=0 because only
> > headroom is missing, but after the call tailroom would be less than
> > before the call.
> > 
> > Why don't we juste reallocate to this size:
> > 
> > MAX(current_alloc_size, nhead + ntail + current_end - current_head)
> 
> Hmm, 
> 
> this changes nothing assuming current_end == skb_end_offset(skb)
> and current_head = skb->head
> 
> Not sure what you mean.

Following patch maybe ...

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index cdc2859..f6c1f52 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1053,11 +1053,22 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 {
 	int i;
 	u8 *data;
-	int size = nhead + skb_end_offset(skb) + ntail;
+	unsigned int tail_offset = skb_tail_pointer(skb) - skb->head;
+	int size = nhead + ntail;
 	long off;
 
 	BUG_ON(nhead < 0);
 
+	/* callers using nhead == 0 and ntail == 0 wants to get a fresh copy,
+	 * so allocate same amount of memory (skb_end_offset)
+	 * For others, they want extra head or tail against the currently
+	 * used portion of header (skb->head -> skb_tail_pointer).
+	 * But we dont shrink the head.
+	 */
+	if (size)
+		size += tail_offset;
+	size = max_t(int, size, skb_end_offset(skb));
+
 	if (skb_shared(skb))
 		BUG();
 
@@ -1074,7 +1085,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	/* Copy only real data... and, alas, header. This should be
 	 * optimized for the cases when header is void.
 	 */
-	memcpy(data + nhead, skb->head, skb_tail_pointer(skb) - skb->head);
+	memcpy(data + nhead, skb->head, tail_offset);
 
 	memcpy((struct skb_shared_info *)(data + size),
 	       skb_shinfo(skb),


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