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

Eric Dumazet
2012-07-14T08:20:53Z

On Sat, 2012-07-14 at 16:04 +0800, Hillf Danton wrote:
> On Sat, Jul 14, 2012 at 1:18 AM, Johannes Truschnigg
> <johannes@truschnigg.info> wrote:
> > Hello good people of linux-kernel.
> >
> > I've been bothered by silent data corruption from my personal fileserver - no
> > matter the Layer 7 protocol used, huge transfers sporadically ended up damaged
> > in-flight. I used Samba/CIFS, NFS(v4, via TCP), Apache httpd 2.2, thttpd,
> > python and netcat to verify this.
> >
> > I think I managed to track down the culprit: as soon as I disable sendfile()
> > for all programs that support such a configuration (netcat, afaik, won't ever
> > use sendfile() to transmit data over a socket, so the problem was never
> > reproducible there in the first place), everything reverts to perfect and
> > proper working condition.
> >
> > I've been experiencing this problem with vanilla kernel releases from the 3.3
> > up until 3.4.0 series. I do not know if it also occurs with earlier releases,
> > but I can verify if that is useful. I set up the environment for a minimal
> > kind of testcase (a large ISO image file available from the server's local
> > filesystem, as well as from a mounted NFS export - once via lo, and once via
> > br0/eth0), and proceeded to do the following:
> >
> > i=0; for i in {1..100}
> > do
> > echo "pass $i:"; sync; echo 3 > /proc/sys/vm/drop_caches
> > cmp -b /mnt/nfs-test/lo/tmp/X15-65741.iso /srv/files/pub/tmp/X15-65741.iso
> > done
> >
> > I then rotated the source of the data, and tested the network-mount against
> > the loopback-mount, as well as the network-mount against the local filesystem.
> >
> > Computing the file's md5sum in a loop whilst dropping caches after each
> > iteration by reading it directly from its location in the filesystem produces
> > the very same hash every time - I therefore think it's safe to assume the
> > corruption is introduced when traversing the networking stack. The hash also
> > does not change if I repeadetly compute the md5sum of the file as transferred
> > by, e. g., Apache httpd or smbd with sendfile explicitly disabled.
> >
> > Please take a look at the attachment to see the actual output of the above
> > script. It does not matter if I do an actual transfer over the network from my
> > server to one of its clients (I verified the problem with two different client
> > machines, one even running Windows), or if the server is both source and
> > destination of the transfer - as long as sendfile is involed, some of the data
> > will always become garbled sooner or later. That also leads me to believe that
> > my internetworking devices (my switch in particular) is working just fine;
> > testing bulky transfers from one host to another confirms this insofar as thus
> > all data makes it through unscathed.
> >
> > As soon as I switch off sendfile-support (in, e. g. Samba or Apache httpd), I
> > can run a series of thousands and more transfers, and not experience any
> > corruption at all. Whenever the data gets fubared, there is no hint at
> > anything fishy going on in the debug ringbuffer - curruption takes place in
> > total silence.
> >
> > The system in question has an Intel Pro/1000 PCI-e NIC for doing the networked
> > file transfers, and is backed by a md RAID5-Array with LVM2 on top. The 4GB of
> > system memory (ECC-enabled UDIMM) are operating in S4ECD4ED mode as reported
> > by EDAC, and there are no reported errors. The CPU I have installed is an AMD
> > Athlon II X2 245e on an ASUS M4A88TD-M/USB3 Motherboard. It's running Gentoo
> > for amd64. The box can run prime96 in torture mode and linpack just fine for
> > days - I'm therefore assuming the hardware to be working correctly.
> >
> > I have attached my kernel's config (from 3.4.0, as that's the image that I
> > have running right now) attached for sake of completeness, as well as some
> > information for you to see how I tested, and what these tests actually
> > produced. If you need any other information to help track this down, please
> > let me know.
> >
> > If you decide to answer please keep me CC'd, as I'm not subscribed to this
> > list.
> >
> > Just in case the numerous attachments get scrubbed/removed, I've also uploaded
> > them to http://johannes.truschnigg.info/tmp/sendfile_data_corruption/
> >
> > Thanks for reading, and have a nice weekend everyone :)
> >
>
> Is the above corruption related to the one below?
>
>
> On Tue, Jul 3, 2012 at 8:02 AM, Willy Tarreau <w@1wt.eu> wrote:
> >
> > In fact it has been true zero copy in 2.6.25 until we faced a large
> > amount of data corruption and the zero copy was disabled in 2.6.25.X.
> > Since then it remained that way until you brought your patches to
> > re-instantiate it.

Might be, or not (could be a NIC bug)

Please Johannes could you try latest kernel tree ?