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Date: Wed, 07 May 2014 03:07:53 +1000
From: Steven Haigh <netwiz@....id.au>
To: Sander Eikelenboom <linux@...elenboom.it>,
Zoltan Kiss <zoltan.kiss@...rix.com>
CC: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
Ian Campbell <Ian.Campbell@...rix.com>,
Eric Dumazet <eric.dumazet@...il.com>, xen-devel@...ts.xen.org
Subject: Re: [3.15-rc3] Bisected: xen-netback mangles packets between two
guests on a bridge since merge of "TX grant mapping with SKBTX_DEV_ZEROCOPY
instead of copy" series.
On a related note, I'm seeing this on an Arch Linux DomU and kernel
3.14.2. Its running aria2c bit torrent client.
The Dom0 is EL6 with Xen 4.2.4 / 3.14.2 kernel.
I also lodged this on the Arch bug tracker:
https://bugs.archlinux.org/task/40244
In a nutshell, I see:
[ 7432.398096] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7434.270870] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7434.782199] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7435.184794] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7437.530677] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7437.973905] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7438.383947] xen_netfront: xennet: skb rides the rocket: 19 slots
On 05/05/14 20:19, Sander Eikelenboom wrote:
>
> Hi Zoltan,
>
> This weekend i tried some more things, the summary:
>
>
> 1) It's a PITA to isolate your patches that went into 3.15 (to rule out any other changes) and apply
> them to 3.14.2, which is tested and worked ok. Could you put up a git tree
> somewhere and rebase your patch series on 3.14.2 for testing ?
>
> 2) Does the test suite you are using also has tests verifying that the content of packets isn't altered ?
>
> 3) It's possible to simplify the test case to a apache webdav server and a
> simple curl put, this simplifies testing and puts ssl and duplicity out of the equation.
>
> 4) There seem to be (at least) two, from the eye of it, separate issues with
> netback / netfront.
> a) Assumption that "An upstream guest shouldn't be able to send 18 slots" is
> false, which probably triggers the netback tx_frag_overflow case.
> b) Corruption of packet content when:
> - sending packets between guests on the same routed network bridge,
> - sending packets between host (dom) and guest goes ok.
> c) Both a and b are regressions from 3.14(.2), although at least a) seems just
> uncovering a latent bug revealed by changed semantics.
>
> 5) Test outcome
>
> --
> Sander
>
> Ad 1) git tree somewhere and rebase your patch series on 3.14.2:
> This is of course unless you are able to trigger this yourself and debug it with the simplified testcase described in (3).
>
> Ad 3) simplify the test case:
> My current setup:
> - working: host kernel 3.14.2 and guest kernels all 3.15-rc4 on Debian wheezy
> - not working: host and guest kernels all 3.15-rc4 on Debian wheezy (.config attached)
> - not working: host and guest kernels all 3.15-rc4 + Eric's patch on Debian wheezy (.config attached)
>
> - guests are on a routed bridge (normal linux kernel bridge which is routed
> with eth0 and eth1.
> - receiving guest has apache 2.2 running with mod_dav.
>
> - test:
> - create a 100mb testfile with a pattern (used perl script is attached)
> - Use curl in dom0 or in the sending guest to send the testfile:
> curl --upload-file testfile.bin http://webdav-guest/storagelocation/
> - check the md5sum of testfile.bin on both sender and receiver
>
> Ad 4a) Assumption that "An upstream guest shouldn't be able to send 18 slots":
> - xen-netfront does this slot check in "xennet_start_xmit":
> slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
> xennet_count_skb_frag_slots(skb);
> if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> net_alert_ratelimited(
> "xennet: skb rides the rocket: %d slots\n", slots);
> goto drop;
> }
>
> - The "MAX_SKB_FRAGS + 1" was changed due to: http://www.gossamer-threads.com/lists/xen/devel/266980,
> but it doesn't seem to be the proper solution.
> - So your assumption doesn't hold, MAX_SKB_FRAGS==17, so 18 slots can come through.
> - On 3.15-rc4 i now started to see this warning getting triggered and packets dropped, i don't see this on 3.14.2:
> [ 118.526583] xen_netfront: xennet: skb rides the rocket: 19 slots | skb_shinfo(skb)->nr_frags: 3, len: 186, offset: 4070, skb->len: 62330, skb->data_len: 62144, skb->truesize: 63424, np->tx.sring->rsp_prod: 21434, np->tx.rsp_cons: 21434 DIV_ROUND_UP(offset + len, PAGE_SIZE): 2
> - So probably some change in semantics makes this thing popup again.
> - What i don't understand is why in:
> xen-netfront this slots check is done when the skb is already dequeued (so dropping is the only thing left to do),
> while in xen-netback it is done before the packet is dequeued (which now seems to work correct since the fixup of Paul to 3.14)
>
> - so your assumption isn't true, but it seems netfront needs to be fixed for that.
>
> - A lot of the (slot) checking logic and frag handling seems to be about the same in xen-netfront and xen-netback, although they seem to have diverted
> somewhat, wouldn't it make sense to put a lot of the generic helper functions in a xen-netcommon.c and share them ?
>
> Ad 4b) Corruption of packet content:
> - The dom0 case doesn't use zerocopy (tx_zerocopy_success: 0 && tx_frag_overflow: 0)
> - I'm getting less convinced it's (directly) coupled to (4a) and the tx_frag_overflow case, although they can occur at about the same time, it
> doesn't necesarrily, the testfile is also corrupt when there is no tx_frag_overflow reported for both vifs:
> ethtool -S vif2.0 (sender)
> NIC statistics:
> rx_gso_checksum_fixup: 0
> tx_zerocopy_sent: 25705
> tx_zerocopy_success: 25538
> tx_zerocopy_fail: 167
> tx_frag_overflow: 0
>
> ethtool -S vif1.0 (receiver)
> NIC statistics:
> rx_gso_checksum_fixup: 0
> tx_zerocopy_sent: 246916
> tx_zerocopy_success: 1
> tx_zerocopy_fail: 246915
> tx_frag_overflow: 0
>
>
> Ad 5) The test described in (3) results into (repeated 5 times each) these md5sums for testfile.bin::
> - generated file: fe599e44789799bae5b6db3df9a34e2d
>
> - dom0 3.14.2 - dom0 to guest: fe599e44789799bae5b6db3df9a34e2d
> - dom0 3.14.2 - guest to guest: fe599e44789799bae5b6db3df9a34e2d
>
> - dom0 3.15-rc4 - dom0 to guest: fe599e44789799bae5b6db3df9a34e2d
> - dom0 3.15-rc4 - guest to guest: 2f51d9baad6f7b2c99aa51e14878a55a fb7df5de7d08b6ad24aa9166949de8c9 0c0afc145f4fed9231e4f1ab6243d02f ef83ace3aafd7e57b8b2fbe324d38995 ffab10c9906381415e5697d2c0e05da3
>
> - dom0 3.15-rc4+eric - dom0 to guest: fe599e44789799bae5b6db3df9a34e2d
> - dom0 3.15-rc4+eric - guest to guest: eb8f48c5613478bb0a69a6115570c713 66fc191b4a04ccddd8b926bc2f57c2b9 99891e0397ca119b0cfaea80b0c6b1f0 0899ab428d102791345c67fa4b608b36 4cc2e3badabc465630d8002004fc0fa3
>
> - That's no good for the guest to guest case .. so inspect the received testfile.bin:
> - length is exactly the same .. good
> - beginning and ending magic strings are there .. good
> - the md5sums differ every time .. no good
> - diff the files to see what is different (one diff from the hexdumps is attached):
> - although the byte counting strings should be unique, in the received testfile.bin they are not, for example:
> grep -i -n 76234752 testfile2.hex
> 4764674:048b4010 20 20 20 20 20 20 20 20 37 36 32 33 34 37 35 32 | 76234752|
> 4764930:048b5010 20 20 20 20 20 20 20 20 37 36 32 33 34 37 35 32 | 76234752|
>
> - So what do we have so far:
> - it look likes all packet metadata is correct, so no warnings or errors from the network stack.
> - only the actual payload gets mangled (otherwise i would have expected warnings from the network stack)
> - it seems to only get mangled when it is travelling "xen-netfront -> xen-netback -> linux netw. bridge -> xen-netback -> xen-netfront".
> - it seems NOT to get mangled when it is travelling "xen-netback -> xen-netfront" only.
> - it's not random corruption, it seems data from older/other frags/packets is used instead of the right data.
> - and a simple test case ... so i hope you can reproduce.
>
> --
> Sander
>
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@...ts.xen.org
> http://lists.xen.org/xen-devel
>
--
Steven Haigh
Email: netwiz@....id.au
Web: http://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
Fax: (03) 8338 0299
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