netdev - Re: [PATCH 2/3] tg3: Fix tx_pending checks for tg3_tso

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Message-Id: <20140821.212514.4787122329921320.davem@davemloft.net>
Date:	Thu, 21 Aug 2014 21:25:14 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bpoirier@...e.de
Cc:	mchan@...adcom.com, prashant@...adcom.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] tg3: Fix tx_pending checks for tg3_tso_bug

From: Benjamin Poirier <bpoirier@...e.de>
Date: Thu, 21 Aug 2014 14:59:11 -0700

> Ah, now I understand the reason for the * 3 in
> 	u32 frag_cnt_est = skb_shinfo(skb)->gso_segs * 3;
> 
> 	/* Estimate the number of fragments in the worst case */
> but that is not really the "worst case". It's not forbidden to have more than
> two frags per skb output from skb_gso_segment(). I've kept this estimation
> approach but I've added code to validate the estimation or else linearize the
> skb.

This is a common situation drivers run into, and there have been a few
notable situations in virtualization drivers recently.  Although in
those cases the problems arise from the fact that if an SKB fragment
is a compound page, this can evaluate to requiring multiple
descriptors, one for each 4K segment within that fragment.

Anyways, the point I wanted to make is that you shouldn't do anything
too complicated to handle all of this.  And I think your conclusion
to linearize if the estimation fails is a good one.
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