netdev - Re: PROBLEM: Silent data corruption when using sendfile()

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Date:	Sat, 14 Jul 2012 12:44:41 +0200
From:	Willy Tarreau <w@....eu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Johannes Truschnigg <johannes@...schnigg.info>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org,
	Linux-Netdev <netdev@...r.kernel.org>
Subject: Re: PROBLEM: Silent data corruption when using sendfile()

On Sat, Jul 14, 2012 at 12:33:24PM +0200, Eric Dumazet wrote:
> On Sat, 2012-07-14 at 12:13 +0200, Johannes Truschnigg wrote:
> > On Sat, Jul 14, 2012 at 10:31:36AM +0200, Willy Tarreau wrote:
> > > > Please Johannes could you try latest kernel tree ?
> > > 
> > > It would be useful, especially given the amount of changes you performed
> > > in this area in latest version, it could be very possible that this new
> > > bug got fixed as a side effect !
> > 
> > I upgraded to 3.4.4 (identical config as the 3.4.0 build I've been running)
> > and what can I say - the problem really seems to have disappeared. I performed
> > about 3700 iterations of my previos tests over the night, and the data always
> > turned out to be OK, not a single byte turned out kaput!
> > 
> > I wish I would have tested that earlier, and spared you the noise... well,
> > maybe someone who runs into a similar problem in the future will have this
> > discovery save her/him some time and headaches and make her/him just upgrade
> > kernels :)
> > 
> > Thanks a lot for your polite and quick responses!
> > 
> 
> Nice to hear. Now we should make sure we have all needed fixes for prior
> stable kernels as well !
> 
> Still trying to understand the issue, since I thought I only did
> optimizations, not bug fixes. So maybe real bug is still there but its
> probability of occurrence lowered enough to not hit your workload.

Please note that Johannes tested 3.4.4 while your changes are in 3.5-rc.

I'm wondering whether this patch merged into 3.4.2 one has an impact on
sendfile :

  commit b642cb6a143da812f188307c2661c0357776a9d0
  Author: Konstantin Khlebnikov <khlebnikov@...nvz.org>
  Date:   Tue Jun 5 21:36:33 2012 +0400

    radix-tree: fix contiguous iterator
    
    commit fffaee365fded09f9ebf2db19066065fa54323c3 upstream.
    
    This patch fixes bug in macro radix_tree_for_each_contig().
    
    If radix_tree_next_slot() sees NULL in next slot it returns NULL, but following
    radix_tree_next_chunk() switches iterating into next chunk. As result iterating
    becomes non-contiguous and breaks vfs "splice" and all its users.

Willy

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