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Date:	Wed, 30 Apr 2014 08:24:06 -0700
From:	Eric Dumazet <>
To:	Sander Eikelenboom <>
Cc:	Zoltan Kiss <>,
	Ian Campbell <>,
	"David S. Miller" <>,,
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 Wed, 2014-04-30 at 12:45 +0200, Sander Eikelenboom wrote:
> Hi Zoltan,
> Your series "TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy", merged into mainline with merge commit 4caeccb4de76440e433a15009636e77d003eb3d6,
> seem to introduce a subtle bug on network traffic between 2 guests on a bridge on the same host.
> I have one guest running apache as webdav server with SSL and another guest that is using that is uploading large files to that webdav server.
> Small requests (some get's and propfind's) seem to work ok, but when the bulk uploading begins it fails with:
> Attempt 1 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL routines:SSL3_READ_BYTES:sslv3 alert bad record mac
> Attempt 2 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL routines:SSL3_READ_BYTES:sslv3 alert bad record mac
> Attempt 3 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL routines:SSL3_READ_BYTES:sslv3 alert bad record mac
> Attempt 4 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL routines:SSL3_READ_BYTES:sslv3 alert bad record mac
> So some how large (probably fragmented) packets can get mangled when from guest to guest on the same host.
> I don't see this with clients that upload large files from external sources.
> Probably if SSL wasn't complaining it would probably be unnoticed for longer and doing some silent corruption.
> I first blamed openssl, since it started around all the latest openssl mayhem and updates, but it turns out it is all xen-netback related again.
> Since these commits break bisectabillity:
>     - 1bb332af4cd889e4b64dacbf4a793ceb3a70445d  (note in commit message && kernel panic)
>     - 62bad3199a4c20505fc36c169deef20b25e17c5f  (kernel panic)
> i stopped bisecting at this point.
> The upside is .. it's 100% reproduceable :-)
> On a sidenote:
>     Although DaveM seems to prefer patches, it would probably be nice to have larger patchsets like this one in a git tree somewhere,
>     for more easy isolated applying to stable kernels, that would improve testing and bisecting things.
>     For example the kernel xen.git tree makes it very easy to pull the devel branches to current stable .. 
>     and even test those *isolated* and *before* the merge window, that greatly reduces the effort of testing and if anything is wrong .. in bisecting things.
>     Another point would be: what *correctness* testing is actually done on the xen-net* patches ?
>     As i suspect this is again about fragmented packets .. that doesn't seem to be included in any test case while it actually seems to be a case which is hard to get right...
> --
> Sander
> The bisection log:
> git bisect start
> # bad: [cd6362befe4cc7bf589a5236d2a780af2d47bcc9] Merge git://
> git bisect bad cd6362befe4cc7bf589a5236d2a780af2d47bcc9
> # good: [df69491b7d1550137507a7eb5f2fc5dce0c1e534] Merge branch 'xen-netback'
> git bisect good df69491b7d1550137507a7eb5f2fc5dce0c1e534
> # good: [c12e69c6aaf785fd307d05cb6f36ca0e7577ead7] Merge tag 'staging-3.15-rc1' of git://
> git bisect good c12e69c6aaf785fd307d05cb6f36ca0e7577ead7
> # bad: [49c0ca17ee8dd3530f688052d4eb2ae6d3e55119] Merge branch 'master' of git:// into for-davem
> git bisect bad 49c0ca17ee8dd3530f688052d4eb2ae6d3e55119
> # good: [370c5acef0326db3e8c10d42b941289c9c887a4a] Merge branch 'for-upstream' of git://
> git bisect good 370c5acef0326db3e8c10d42b941289c9c887a4a
> # good: [ec5709403e6893acb4f7ca40514ebd29c3116836] net/mlx4_en: Use union for BlueFlame WQE
> git bisect good ec5709403e6893acb4f7ca40514ebd29c3116836
> # bad: [69bfb110fd58185df99a7dbe92a14c0d7ada764f] i40e: cleanup strings
> git bisect bad 69bfb110fd58185df99a7dbe92a14c0d7ada764f
> # bad: [aeb12c5ef7cb08d879af22fc0a56cab9e70689ea] gianfar: Separate out the Tx interrupt handling (Tx NAPI)
> git bisect bad aeb12c5ef7cb08d879af22fc0a56cab9e70689ea
> # bad: [389400428953bb002b173fa07d16d7a6f120843f] Merge tag 'rxrpc-devel-20140304' of git://
> git bisect bad 389400428953bb002b173fa07d16d7a6f120843f
> # good: [5a8a1ab74dce1b50fe27745df477c502aec987eb] be2net: do external loopback test only when it is requested
> git bisect good 5a8a1ab74dce1b50fe27745df477c502aec987eb
> # skip: [1bb332af4cd889e4b64dacbf4a793ceb3a70445d] xen-netback: Add stat counters for zerocopy
> git bisect skip 1bb332af4cd889e4b64dacbf4a793ceb3a70445d
> # skip: [e86800f9201d35b6b2aac1583a9bf9e3a0b0c70d] Merge branch '6lowpan'
> git bisect skip e86800f9201d35b6b2aac1583a9bf9e3a0b0c70d
> # good: [31c70d5956fc3d1abf83e9ab5e1d8237dea59498] l2tp: keep original skb ownership
> git bisect good 31c70d5956fc3d1abf83e9ab5e1d8237dea59498

This looks like skb->data_len is not properly set 

(Small note : 
alloc_page(GFP_ATOMIC|__GFP_COLD) should be a plain
alloc_page(GFP_ATOMIC) : We prefer a page that has been used recently to
avoid cache misses if possible.

This probably confuses skb_try_coalesce()

My guess is xenvif_get_requests() forgets to init nskb->len /
nskb->data_len for the skb that is attached to frag_list

So the "skb->data_len += nskb->len;" at line 1353 is probably a nop.

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