netdev - Re: [PATCH] tcp: splice as many packets as possible at once

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Date:	Wed, 14 Jan 2009 10:47:16 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	herbert@...dor.apana.org.au, zbr@...emap.net, dada1@...mosbay.com,
	w@....eu, ben@...s.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [PATCH] tcp: splice as many packets as possible at once

On Wed, Jan 14, 2009 at 02:06:37AM -0800, David Miller wrote:
> From: Jarek Poplawski <jarkao2@...il.com>
> Date: Wed, 14 Jan 2009 09:42:16 +0000
> 
> > On Wed, Jan 14, 2009 at 01:29:19AM -0800, David Miller wrote:
> > > From: Jarek Poplawski <jarkao2@...il.com>
> > > Date: Wed, 14 Jan 2009 08:53:08 +0000
> > > 
> > > > Actually, I still think my second approach (the PageSlab) is probably
> > > > (if tested) the easiest for now, because it should fix the reported
> > > > (Willy's) problem, without any change or copy overhead for splice to
> > > > file (which could be still wrong, but not obviously wrong).
> > > 
> > > It's a simple fix, but as Herbert stated it leaves other ->sendpage()
> > > implementations exposed to data corruption when the from side of the
> > > pipe buffer is a socket.
> > 
> > I don't think Herbert meant other ->sendpage() implementations, but I
> > could miss something.
> 
> I think he did :-)

I hope Herbert will make it clear.

> 
> Or, more generally, he could have been referring to splice pipe
> outputs.  All of these things grab references to pages and
> expect that to keep the underlying data from being reallocated.
> 
> That doesn't work for this skb->data case.
> 
> > > That, to me, is almost worse than a bad fix.
> > > 
> > > It's definitely worse than a slower but full fix, which the copy
> > > patch is.
> > 
> > Sorry, I can't see how this patch could make sendpage worse.
> 
> Because that patch only fixes TCP's ->sendpage() implementation.

Yes, it fixes (I hope) the only reported implementation.

> 
> There are others out there which could end up experiencing similar
> data corruption.

Since this fix is very simple (and IMHO safe) it could be probably
used elsewhere too, but I doubt we should care at the moment.

Jarek P.
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