Re: [Xen-devel] PATastic fun

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 25.02.2013 04:15, Liu, Jinsong wrote:
> Konrad Rzeszutek Wilk wrote:
>> On Fri, Feb 22, 2013 at 02:54:16PM +0100, Stefan Bader wrote:
>>> Hi Konrad,
>>
>> Hey Stefan,
>>>
>>> here is another one from the hm-what? department:
>>
>> Heh - the really good-bug-hunting one. Lets also include Jinsong as
>> he has been tracking a similar one with mcelog.
>>>
>>> Colin discovered that running the attached program with the fork
>>> active (e.g. "./mmap-example -f 0x10000", the address can be that or
>>> iomem) this triggers the following weird messages: 
>>>
>>> [ 6824.453724] mmap-example:3481 map pfn expected mapping type
>>> write-back for [mem 0x00010000-0x00010fff], got uncached-minus
>>> [ 6824.453776] ------------[ cut here ]------------
>>> [ 6824.453796] WARNING: at
>>> /build/buildd/linux-3.8.0/arch/x86/mm/pat.c:774
>>> untrack_pfn+0xb8/0xd0() ... [ 6824.453920] Pid: 3481, comm:
>>> mmap-example Tainted: GF 
>>> 3.8.0-6-generic #13-Ubuntu
>>> [ 6824.453926] Call Trace:
>>> [ 6824.453944]  [<ffffffff8105879f>] warn_slowpath_common+0x7f/0xc0
>>> [ 6824.453954]  [<ffffffff810587fa>] warn_slowpath_null+0x1a/0x20
>>> [ 6824.453963]  [<ffffffff8104bcc8>] untrack_pfn+0xb8/0xd0
>>> [ 6824.453975]  [<ffffffff81156c1c>] unmap_single_vma+0xac/0x100
>>> [ 6824.453985]  [<ffffffff81157459>] unmap_vmas+0x49/0x90
>>> [ 6824.453995]  [<ffffffff8115f808>] exit_mmap+0x98/0x170
>>> [ 6824.454007]  [<ffffffff810559a4>] mmput+0x64/0x100
>>> [ 6824.454017]  [<ffffffff810560f5>] dup_mm+0x445/0x660
>>> [ 6824.454027]  [<ffffffff81056d9f>]
>>> copy_process.part.22+0xa5f/0x1510 [ 6824.454038] 
>>> [<ffffffff81057931>] do_fork+0x91/0x350 [ 6824.454048] 
>>> [<ffffffff81057c76>] sys_clone+0x16/0x20 [ 6824.454060] 
>>> [<ffffffff816ccbf9>] stub_clone+0x69/0x90 [ 6824.454069] 
>>> [<ffffffff816cc89d>] ? system_call_fastpath+0x1a/0x1f [ 6824.454076]
>>> ---[ end trace 4918cdd0a4c9fea4 ]--- 
>>>
>>> I found that this is related to your bandaid patch
>>>
>>> commit 8eaffa67b43e99ae581622c5133e20b0f48bcef1
>>> Author: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>> Date:   Fri Feb 10 09:16:27 2012 -0500
>>>
>>>     xen/pat: Disable PAT support for now.
>>>
>>> I just do not understand how this happens. From the trace it seems
>>> the fork 
>>> fails when duplicating the VMAs (dup_mm calls mmput on failure). So
>>> maybe the 
>>> warning is just related to this. So primarily the question is how on
>>> fork the _PAGE_PCD bit can become set? That and _PAGE_PWT are
>>> cleared from the supported 
>>> mask by the patch, so somehow I would think nothing should be able
>>> to set it... 
>>> But apparently not so.
>>> Not sure it is a big deal since I never saw this in normal operation
>>> and it 
>>> seems to be ok when unapping before doing the fork. It is just plain
>>> odd. 
>>
>> Jinsong mentioned that there is some oddity with the MTRR. Somehow the
>> ranges are swapped or not correct. Jinsong, could you shed some light
>> on what you have found so far?
>>
> 
> Yes, Sander once also reported a similar weird warning when start mcelog 
> daemon, as attached.
> 
> Basically, it occurs when mcelog user daemon start, 
> do_fork
>   --> copy_process
>     --> dup_mm
>       --> dup_mmap
>         --> copy_page_range
>           --> track_pfn_copy
>             --> reserve_pfn_range
>               --> line 624: flags != want_flags
> It comes from different memory types of page table (_PAGE_CACHE_WB) and mtrr 
> (_PAGE_CACHE_UC_MINUS).
> 
> However, why it get different memory types from page table and mtrr is still 
> unclear, reproducing the bug is difficult and unstable.
> 
> Thanks,

Ok, so this seems to take the same code paths. As for the test program, it fails
on duplicating some mmap on a fork. The test program does this all the time
(except the backtrace warning which is warn_once).
So you say, the UC- comes from the MTRR side... Hm, have to look at that.

Thanks,
Stefan

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