netdev - RE: [PATCH net-next 0/4] enic: improve rq buff allocation and reduce dma mapping

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Date:	Mon, 2 Feb 2015 23:19:31 +0530 (IST)
From:	Govindarajulu Varadarajan <_govind@....com>
To:	David Laight <David.Laight@...LAB.COM>
cc:	'Govindarajulu Varadarajan' <_govind@....com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"ssujith@...co.com" <ssujith@...co.com>,
	"benve@...co.com" <benve@...co.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"ben@...adent.org.uk" <ben@...adent.org.uk>
Subject: RE: [PATCH net-next 0/4] enic: improve rq buff allocation and reduce
 dma mapping

On Mon, 2 Feb 2015, David Laight wrote:
> Two questions:
> 1) How are you handling the skb's true_size ?

skb->true_size is set to ksize(data) for data allocated using alloc_page and
kmalloc. For frags we set it to size of frag. Check the function build_skb()

> 2) Memory fragmentation could easily make the allocation of 32k fail.
>

With huge memory these days order-3 allocation failure is quite rare. In my
testing on system with 8G memory I have never encountered order-3 failure.
This is probably why __netdev_alloc_frag tries order-3 page allocation first.

If order-3 page allocation fails, we drop to minimum order required for the
given size.
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