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Re: [Xen-devel] [xen-unstable test] 123379: regressions - FAIL

  • To: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Wed, 13 Jun 2018 10:52:53 +0200
  • Autocrypt: addr=jgross@xxxxxxxx; prefer-encrypt=mutual; keydata= xsBNBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB AAHNHkp1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmRlPsLAeQQTAQIAIwUCU4xw6wIbAwcL CQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJELDendYovxMvi4UH/Ri+OXlObzqMANruTd4N zmVBAZgx1VW6jLc8JZjQuJPSsd/a+bNr3BZeLV6lu4Pf1Yl2Log129EX1KWYiFFvPbIiq5M5 kOXTO8Eas4CaScCvAZ9jCMQCgK3pFqYgirwTgfwnPtxFxO/F3ZcS8jovza5khkSKL9JGq8Nk czDTruQ/oy0WUHdUr9uwEfiD9yPFOGqp4S6cISuzBMvaAiC5YGdUGXuPZKXLpnGSjkZswUzY d9BVSitRL5ldsQCg6GhDoEAeIhUC4SQnT9SOWkoDOSFRXZ+7+WIBGLiWMd+yKDdRG5RyP/8f 3tgGiB6cyuYfPDRGsELGjUaTUq3H2xZgIPfOwE0EU4xwFgEIAMsx+gDjgzAY4H1hPVXgoLK8 B93sTQFN9oC6tsb46VpxyLPfJ3T1A6Z6MVkLoCejKTJ3K9MUsBZhxIJ0hIyvzwI6aYJsnOew cCiCN7FeKJ/oA1RSUemPGUcIJwQuZlTOiY0OcQ5PFkV5YxMUX1F/aTYXROXgTmSaw0aC1Jpo w7Ss1mg4SIP/tR88/d1+HwkJDVW1RSxC1PWzGizwRv8eauImGdpNnseneO2BNWRXTJumAWDD pYxpGSsGHXuZXTPZqOOZpsHtInFyi5KRHSFyk2Xigzvh3b9WqhbgHHHE4PUVw0I5sIQt8hJq 5nH5dPqz4ITtCL9zjiJsExHuHKN3NZsAEQEAAcLAXwQYAQIACQUCU4xwFgIbDAAKCRCw3p3W KL8TL0P4B/9YWver5uD/y/m0KScK2f3Z3mXJhME23vGBbMNlfwbr+meDMrJZ950CuWWnQ+d+ Ahe0w1X7e3wuLVODzjcReQ/v7b4JD3wwHxe+88tgB9byc0NXzlPJWBaWV01yB2/uefVKryAf AHYEd0gCRhx7eESgNBe3+YqWAQawunMlycsqKa09dBDL1PFRosF708ic9346GLHRc6Vj5SRA UTHnQqLetIOXZm3a2eQ1gpQK9MmruO86Vo93p39bS1mqnLLspVrL4rhoyhsOyh0Hd28QCzpJ wKeHTd0MAWAirmewHXWPco8p1Wg+V+5xfZzuQY0f4tQxvOpXpt4gQ1817GQ5/Ed/wsDtBBgB CAAgFiEEhRJncuj2BJSl0Jf3sN6d1ii/Ey8FAlrd8NACGwIAgQkQsN6d1ii/Ey92IAQZFggA HRYhBFMtsHpB9jjzHji4HoBcYbtP2GO+BQJa3fDQAAoJEIBcYbtP2GO+TYsA/30H/0V6cr/W V+J/FCayg6uNtm3MJLo4rE+o4sdpjjsGAQCooqffpgA+luTT13YZNV62hAnCLKXH9n3+ZAgJ RtAyDWk1B/0SMDVs1wxufMkKC3Q/1D3BYIvBlrTVKdBYXPxngcRoqV2J77lscEvkLNUGsu/z W2pf7+P3mWWlrPMJdlbax00vevyBeqtqNKjHstHatgMZ2W0CFC4hJ3YEetuRBURYPiGzuJXU pAd7a7BdsqWC4o+GTm5tnGrCyD+4gfDSpkOT53S/GNO07YkPkm/8J4OBoFfgSaCnQ1izwgJQ jIpcG2fPCI2/hxf2oqXPYbKr1v4Z1wthmoyUgGN0LPTIm+B5vdY82wI5qe9uN6UOGyTH2B3p hRQUWqCwu2sqkI3LLbTdrnyDZaixT2T0f4tyF5Lfs+Ha8xVMhIyzNb1byDI5FKCb
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
  • Delivery-date: Wed, 13 Jun 2018 08:53:00 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 12/06/18 17:58, Juergen Gross wrote:
> On 08/06/18 12:12, Juergen Gross wrote:
>> On 07/06/18 13:30, Juergen Gross wrote:
>>> On 06/06/18 11:40, Juergen Gross wrote:
>>>> On 06/06/18 11:35, Jan Beulich wrote:
>>>>>>>> On 05.06.18 at 18:19, <ian.jackson@xxxxxxxxxx> wrote:
>>>>>>>>  test-amd64-i386-libvirt-qemuu-debianhvm-amd64-xsm 14 
>>>>>>>> guest-saverestore.2 
>>>>>> I thought I would reply again with the key point from my earlier mail
>>>>>> highlighted, and go a bit further.  The first thing to go wrong in
>>>>>> this was:
>>>>>> 2018-05-30 22:12:49.320+0000: xc: Failed to get types for pfn batch (14 
>>>>>> = Bad address): Internal error
>>>>>> 2018-05-30 22:12:49.483+0000: xc: Save failed (14 = Bad address): 
>>>>>> Internal error
>>>>>> 2018-05-30 22:12:49.648+0000: libxl-save-helper: complete r=-1: Bad 
>>>>>> address
>>>>>> You can see similar messages in the other logfile:
>>>>>> 2018-05-30 22:12:49.650+0000: libxl: 
>>>>>> libxl_stream_write.c:350:libxl__xc_domain_save_done: Domain 3:saving 
>>>>>> domain: domain responded to suspend request: Bad address
>>>>>> All of these are reports of the same thing: xc_get_pfn_type_batch at
>>>>>> xc_sr_save.c:133 failed with EFAULT.  I'm afraid I don't know why.
>>>>>> There is no corresponding message in the host's serial log nor the
>>>>>> dom0 kernel log.
>>>>> I vaguely recall from the time when I had looked at the similar Windows
>>>>> migration issues that the guest is already in the process of being cleaned
>>>>> up when these occur. Commit 2dbe9c3cd2 ("x86/mm: silence a pointless
>>>>> warning") intentionally suppressed a log message here, and the
>>>>> immediately following debugging code (933f966bcd x86/mm: add
>>>>> temporary debugging code to get_page_from_gfn_p2m()) was reverted
>>>>> a little over a month later. This wasn't as a follow-up to another patch
>>>>> (fix), but following the discussion rooted at
>>>>> https://lists.xenproject.org/archives/html/xen-devel/2017-06/msg00324.html
>>>> That was -ESRCH, not -EFAULT.
>>> I've looked a little bit more into this.
>>> As we are seeing EFAULT being returned by the hypervisor this either
>>> means the tools are specifying an invalid address (quite unlikely)
>>> or the buffers are not as MAP_LOCKED as we wish them to be.
>>> Is there a way to see whether the host was experiencing some memory
>>> shortage, so the buffers might have been swapped out?
>>> man mmap tells me: "This implementation will try to populate (prefault)
>>> the whole range but the mmap call doesn't fail with ENOMEM if this
>>> fails. Therefore major faults might happen later on."
>>> And: "One should use mmap(2) plus mlock(2) when major faults are not
>>> acceptable after the initialization of the mapping."
>>> With osdep_alloc_pages() in tools/libs/call/linux.c touching all the
>>> hypercall buffer pages before doing the hypercall I'm not sure this
>>> could be an issue.
>>> Any thoughts on that?
>> Ian, is there a chance to dedicate a machine to a specific test trying
>> to reproduce the problem? In case we manage to get this failure in a
>> reasonable time frame I guess the most promising approach would be to
>> use a test hypervisor producing more debug data. If you think this is
>> worth doing I can write a patch.
> Trying to reproduce the problem in a limited test environment finally
> worked: doing a loop of "xl save -c" produced the problem after 198
> iterations.
> I have asked a SUSE engineer doing kernel memory management if he
> could think of something. His idea is that maybe some kthread could be
> the reason for our problem, e.g. trying page migration or compaction
> (at least on the test machine I've looked at compaction of mlocked
> pages is allowed: /proc/sys/vm/compact_unevictable_allowed is 1).
> In order to be really sure nothing in the kernel can temporarily
> switch hypercall buffer pages read-only or invalid for the hypervisor
> we'll have to modify the privcmd driver interface: it will have to
> gain knowledge which pages are handed over to the hypervisor as buffers
> in order to be able to lock them accordingly via get_user_pages().
> While this is a possible explanation of the fault we are seeing it might
> be related to another reason. So I'm going to apply some modifications
> to the hypervisor to get some more diagnostics in order to verify the
> suspected kernel behavior is really the reason for the hypervisor to
> return EFAULT.

I was lucky. Took only 39 iterations this time.

The debug data confirms the theory that the kernel is setting the PTE to
invalid or read only for a short amount of time:

(XEN) fixup for address 00007ffb9904fe44, error_code 0002:
(XEN) Pagetable walk from 00007ffb9904fe44:
(XEN)  L4[0x0ff] = 0000000458da6067 0000000000019190
(XEN)  L3[0x1ee] = 0000000457d26067 0000000000018210
(XEN)  L2[0x0c8] = 0000000445ab3067 0000000000006083
(XEN)  L1[0x04f] = 8000000458cdc107 000000000001925a
(XEN) Xen call trace:
(XEN)    [<ffff82d0802abe31>] __copy_to_user_ll+0x27/0x30
(XEN)    [<ffff82d080272edb>] arch_do_domctl+0x5a8/0x2648
(XEN)    [<ffff82d080206d5d>] do_domctl+0x18fb/0x1c4e
(XEN)    [<ffff82d08036d1ba>] pv_hypercall+0x1f4/0x43e
(XEN)    [<ffff82d0803734a6>] lstar_enter+0x116/0x120

The page was writable again when the page walk data has been collected,
but A and D bits still are 0 (which should not be the case in case the
kernel didn't touch the PTE, as the hypervisor read from that page some
instructions before the failed write).

Starting with the Xen patches now...


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