From mboxrd@z Thu Jan 1 00:00:00 1970 From: Grant Edwards Subject: Change in alloc_skb() behavior in 3.2+ kernels? Date: Wed, 6 Jun 2012 18:32:57 +0000 (UTC) Message-ID: To: netdev@vger.kernel.org Return-path: Received: from plane.gmane.org ([80.91.229.3]:47850 "EHLO plane.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751634Ab2FFSdX (ORCPT ); Wed, 6 Jun 2012 14:33:23 -0400 Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1ScL2k-0000tb-U6 for netdev@vger.kernel.org; Wed, 06 Jun 2012 20:33:18 +0200 Received: from dsl.comtrol.com ([64.122.56.22]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Wed, 06 Jun 2012 20:33:18 +0200 Received: from grant.b.edwards by dsl.comtrol.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Wed, 06 Jun 2012 20:33:18 +0200 Sender: netdev-owner@vger.kernel.org List-ID: I'm tracking down a problem that appears to be caused by a change in the behavior of alloc_skb() introduced in kernel version 3.2. In kernel versions prior to 3.2, calling alloc_skb(1350), returned an sk_buff with a tailroom of around 1400 bytes (safely below the default Ethernet frame size limit of 1500). In 3.2 and later, calling alloc_skb(1350) returns an sk_buff with a tailroom of about 1850. Why has the "extra" space increased from 60 bytes to 500 bytes? [It's always possible that I've unintentionally changed something in the kernel configs that causes this, but I've tried to build the kernels as identically as possible.] The kernel module that's started failing fills the allocated sk_buff until tailroom() indicates it is full and then sends it. The problem is that sending a packet with a length of 1850 won't work (it's a MAC-layer Ethernet packet). I've found man pages for alloc_skb() from a few years ago that state explicitly that alloc_skb(_size_) will allocate a new sk_buff with no headroom and a tail room of _size_ bytes. This doesn't seem to be the case for recent kernels. Is there any documentation stating what the current behavior is supposed to be? Are callers to alloc_skb() supposed to check the tailroom and reserve() an appropriate number of bytes such that the tailroom is correct? Is the tailroom of the allocated sk_buff guaranteed to be at least as large as the requested size, or does application code also have to check for tailroom less than the requested size? The ultimate question I'm trying to answer is what is the "right" way to allocate an sk_buff that has a size appropriate for an Ethernet frame assuming an MTU of 1500? -- Grant Edwards grant.b.edwards Yow! Let's send the at Russians defective gmail.com lifestyle accessories!