netdev - RE: [PATCH net-next] net: adjust skb->truesize in pskb_expand

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Date:   Fri, 27 Jan 2017 17:24:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>
CC:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "Slava Shwartsman" <slavash@...lanox.com>
Subject: RE: [PATCH net-next] net: adjust skb->truesize in pskb_expand_head()

From: Eric Dumazet
> Sent: 27 January 2017 14:44
...
> > I'm also guessing that extra headroom can be generated by stealing unused tailroom.
> 
> This is already done.
> 
> Quoting
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=87fb4b7b533073eeeaed0b6bf7c
> 2328995f6c075
> 
>     At skb alloc phase, we put skb_shared_info struct at the exact end of
>     skb head, to allow a better use of memory (lowering number of
>     reallocations), since kmalloc() gives us power-of-two memory blocks.

Does that actually have the expected effect?

Allocate an skb for 512 bytes, copy in some data with 64 bytes of headroom.
This is (probably) a 1k memory block with skb_shared_info at the end.

Some code needs to add a header that doesn't fit, calls pskb_expand_head()
to get another 4 bytes.
Since the existing amount of 'tailroom' must be kept kmalloc(1024+4) is called.
This allocates a 2k memory block, again skb_shared_info is put at the end.
So the headroom has been increased by 4 bytes and the tailroom by 1020.

Another layer needs to add another header.
The memory block becomes 4k large.

What have I missed?

	David