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

Willy Tarreau
2012-07-14T10:44:47Z

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@openvz.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