netdev - RE: skbuff truesize incorrect.

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Message-ID: <1400859695.5367.203.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 23 May 2014 08:41:35 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Jim Baxter <jim_baxter@...tor.com>,
	'Bjørn Mork' <bjorn@...k.no>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	"kamal@...onical.com" <kamal@...onical.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>,
	"fw@...len.de" <fw@...len.de>
Subject: RE: skbuff truesize incorrect.

On Fri, 2014-05-23 at 15:30 +0000, David Laight wrote:
> From: Eric Dumazet
> ...
> > TCP stack uses fast clones, and current stack gives them a truesize of
> > 2048 + sizeof(sk_buff), while it really should be 2048 +
> > 2*sizeof(sk_buff)
> > 
> > Luckily, GSO/TSO tends to reduce the error, as skbs overhead is lower.
> 
> Doesn't that affect the tx side - where the truesize doesn't matter as much?

Its not a matter of tx or rx, but percentage of error.

If truesize accounting is wrong by 10%, its not a big deal, because we
normally limit tcp_mem[] to about 16% of available physical memory.

Using 16% of physical memory instead of 16% should not really matter.

Now, if the truesize is wrong by 1600%, then its pretty clear we can
consume all the meory.



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