Re: Change in alloc_skb() behavior in 3.2+ kernels?

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2012-06-06 at 18:32 +0000, Grant Edwards wrote:
> 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?
> 

Because of kmalloc-2048 being used. Previous kernels were losing this
space. We are now able to expand some packets without extra
re-allocation/copy.

> [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).

This code seems buggy.

> 
> 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?
> 

If you allocate skbs with 1500 bytes, you probably should check skb->len
more than tailroom...

> 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?
> 

I dont know what to answer. Could you point the code in question ?