netdev - Re: [PATCH] net: allow configuration of the size of page in __netdev_alloc

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Date:	Wed, 24 Oct 2012 15:02:06 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH] net: allow configuration of the size of page in
 __netdev_alloc_frag

On Wed, 2012-10-24 at 14:30 +0100, Eric Dumazet wrote:
> It seems to me its a driver issue, for example
> drivers/net/xen-netfront.c has assumptions that can be easily fixed.

The netfront ->head thing is a separate (although perhaps related)
issue, I intended to fix along the same lines as the previous netback
except for some unfathomable reason I haven't been able to reproduce the
problem with netfront -- I've no idea why though since it seems like it
should be a no brainer!

> Why skb->head can be on order-1 or order-2 pages and this is working ?

skb->head being order 1 or 2 isn't working for me. The driver I'm having
issues with which caused me to create this particular patch is the tg3
driver (although I don't think this is by any means specific to tg3).

For the ->head the tg3 driver does:
        mapping = pci_map_single(tp->pdev, skb->data, len, PCI_DMA_TODEVICE);
while for the frags it does:
        mapping = skb_frag_dma_map(&tp->pdev->dev, frag, 0, len, DMA_TO_DEVICE);

This ought to do the Right Thing but doesn't seem to be working. Konrad
suspected an issue with the swiotlb's handling of order>0 pages in some
cases. As I said in the commit message he is looking into this issue.

My concern however was that even once the swiotlb is fixed to work right
the effect of pci_map_single on a order>0 page is going to be that the
data gets bounced into contiguous memory -- that is a memcpy which would
undo the benefit of having allocating large pages to start with. So I
figured that in such cases we'd be better off just using order 0
allocations to start with.

Ian.

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