netdev - Re: [PATCH] net: use hardware buffer pool to allocate skb

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Date:	Thu, 16 Oct 2014 09:57:37 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	"Jiafei.Pan@...escale.com" <Jiafei.Pan@...escale.com>,
	David Miller <davem@...emloft.net>,
	"jkosina@...e.cz" <jkosina@...e.cz>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"LeoLi@...escale.com" <LeoLi@...escale.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] net: use hardware buffer pool to allocate skb

On Thu, 2014-10-16 at 08:28 -0700, Alexander Duyck wrote:

> I think the part you are not getting is that is how buffers are 
> essentially handled now.  So for example in the case if igb the only 
> part we have copied out is usually the header, or the entire frame in 
> the case of small packets.  This has to happen in order to allow for 
> changes to the header for routing and such.  Beyond that the frags that 
> are passed are the buffers that igb is still holding onto.  So 
> effectively what the other device transmits in a bridging/routing 
> scenario is my own net card specified buffer plus the copied/modified 
> header.
> 
> For a brief period igb used build_skb but that isn't valid on most 
> systems as memory mapped for a device can be overwritten if the page is 
> unmapped resulting in any changes to the header for routing/bridging 
> purposes being invalidated.  Thus we cannot use the buffers for both the 
> skb->data header which may be changed and Rx DMA simultaneously.

This reminds me that igb still has skb->truesize underestimation by 100%

If a fragment is held in some socket receive buffer, a full page is
consumed, not 2048 bytes.

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index a21b14495ebd..56ca6c78985e 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -6586,9 +6586,11 @@ static bool igb_add_rx_frag(struct igb_ring *rx_ring,
 	struct page *page = rx_buffer->page;
 	unsigned int size = le16_to_cpu(rx_desc->wb.upper.length);
 #if (PAGE_SIZE < 8192)
-	unsigned int truesize = IGB_RX_BUFSZ;
+	unsigned int segsize = IGB_RX_BUFSZ;
+	unsigned int truesize = PAGE_SIZE;
 #else
-	unsigned int truesize = ALIGN(size, L1_CACHE_BYTES);
+	unsigned int segsize = ALIGN(size, L1_CACHE_BYTES);
+	unsigned int truesize = segsize;
 #endif
 
 	if ((size <= IGB_RX_HDR_LEN) && !skb_is_nonlinear(skb)) {
@@ -6614,7 +6616,7 @@ static bool igb_add_rx_frag(struct igb_ring *rx_ring,
 	skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page,
 			rx_buffer->page_offset, size, truesize);
 
-	return igb_can_reuse_rx_page(rx_buffer, page, truesize);
+	return igb_can_reuse_rx_page(rx_buffer, page, segsize);
 }
 
 static struct sk_buff *igb_fetch_rx_buffer(struct igb_ring *rx_ring,
 

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