netdev - [PATCH] net: allow pskb_expand_head() to get maximum tailroom

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Message-ID: <1334124519.5300.2246.camel@edumazet-glaptop>
Date:	Wed, 11 Apr 2012 08:08:39 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Marc MERLIN <marc@...lins.org>
Cc:	David Miller <davem@...emloft.net>, Larry.Finger@...inger.net,
	bhutchings@...arflare.com, linux-wireless@...r.kernel.org,
	netdev@...r.kernel.org
Subject: [PATCH] net: allow pskb_expand_head() to get maximum tailroom

Marc Merlin reported many order-1 allocations failures in TX path on its
wireless setup, that dont make any sense with MTU=1500 network, and non
SG capable hardware.

Turns out part of the problem comes from pskb_expand_head() not using
ksize() to get exact head size given by kmalloc(). Doing the same thing
than __alloc_skb() allows more tailroom in skb and can prevent future
reallocations.

As a bonus, struct skb_shared_info becomes cache line aligned.

Reported-by: Marc MERLIN <marc@...lins.org>
Tested-by: Marc MERLIN <marc@...lins.org>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/core/skbuff.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index baf8d28..e598400 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -952,9 +952,11 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 		goto adjust_others;
 	}

-	data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask);
+	data = kmalloc(size + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)),
+		       gfp_mask);
 	if (!data)
 		goto nodata;
+	size = SKB_WITH_OVERHEAD(ksize(data));

 	/* Copy only real data... and, alas, header. This should be
 	 * optimized for the cases when header is void.

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