[lvm-devel] Internal error: Pool read_vg crc mismatch only when running in test environment

Zdenek Kabelac zkabelac at redhat.com
Fri Jun 28 09:14:59 UTC 2013

Dne 27.6.2013 00:49, Tony Asleson napsal(a):
> On 06/26/2013 02:49 AM, Zdenek Kabelac wrote:
>> Dne 26.6.2013 01:28, Tony Asleson napsal(a):
>>> On 06/21/2013 08:53 AM, Zdenek Kabelac wrote:
>>>> Dne 21.6.2013 01:18, Tony Asleson napsal(a):
>>>>> Writing some new unit test cases for my latest liblvm patch set and at
>>>>> the moment I am running into a case where I can run the unit test case
>>>>> against real disk and it works, but if I run it in the test environment
>>>>> with loop back devices I am getting an abort with:
>>>>> "Internal error: Pool read_vg crc mismatch."
>>>>> Any ideas why this error isn't occurring on both?
>>>> This happens - if you have requested 'read-only'  VG struct,
>>>> and you have modifed something in this vgmem pool
>>>> (either writing to struct, or just using vgmem pool for allocation)
>>> My original inquiry was for ideas why I see this on loop back devices
>>> and not actual devices.  This response doesn't seem to match what I am
>>> seeing.
>> It's hard to give advice if I do not see the actual code.
> Patch set was posted a while back.  Specifically the issue I am running
> into has to do with the functionality to list all PVs (including orphan)
> Specific patch is:
> https://www.redhat.com/archives/lvm-devel/2013-May/msg00036.html
>>> The stack trace shows that we are getting this error during an
>>> lvm_vg_open.  I can re-create the error regardless if I open the vg
>>> struct as read-only or read-write.
>>>   From my initial debug it appears that if the vginfo->vg_use_count > 1
>>> (in this case 3) we pass 1 as the second parameter to dm_pool_unlock
>>> which is a crc check of the pool which it finds to be different.  At
>>> this point it would seem I am exacerbating some type of caching bug or
>>> that somewhere along the path I am inadvertently changing the contents
>>> of the vg struct pointer with my latest patches.
>> There is internal debug support for this kind of problems which mprotects
>> vg structure, so any write access to locked vg structure crashes the
>> application and you may look at stack trace.
>> But it needs some hand modification of make.tmpl file (no configure option)
>> Uncomment #DEFS += -DDEBUG_ENFORCE_POOL_LOCKING  and rebuild and retest
>> with
>> unlimited coredump size.
> Thanks for the tip, this helped quite a bit!  I modified the code in the
> above referenced patch to retain the vg pointer when retrieving the list
> of PVs.  The liblvm library uses the vg->vgmem pool to allocate strings
> to be returned to the user when getting information pertaining to a PV.
>   The problem is that the vg gets cached with a crc.  We then allocate
> memory from the vgmem pool and later when we go to clear out the cache
> entry we fail the crc check.  When I enable the
> DEBUG_ENFORCE_POOL_LOCKING we fail after we retrieve a vg from the cache
> and then I try to allocate something from the vgmem pool.
> Your comment about opening the vg "r" vs. "w" still seems incorrect to
> me, but if it is indeed correct then we have a problem with the library
> that precedes any of my changes.  We use the vg->vgmem for many things
> and if this argument is indeed true then all those allocations will be
> causing the vg struct to change, thus causing crc errors if the user is
> allowed to open the vg struct as read-only.  In my code review and
> testing it appears that we aren't hitting this because when we call into
> vg_open we don't have a vgid, so we fail early in lvmcache_get_vg and
> thus we never add them to the cache and thus we never call dm_pool_lock
> on them.
> If you could point out the bit of code that actually determines that we
> mprotect memory based on the user doing an vg open read only vs. read
> write that would be most helpful.  I'm not seeing it at the moment and I
> would like to understand this better.
> Currently I am thinking about just backing out the change to retrieve
> the pv list and use the cmd->mem for allocations instead of the
> vg->vgmem.  Previously I brought up just putting such things on the heap
> and letting the users of the library free them, but that was dismissed
> because existing library users would then have memory leaks.

yeah there could be some design fault which is using mem pool from read-only
volume group.  vg->vgmem  is only used for allocation of objects directly 
related to VG  -  return of some copied buffers is not something which is 
supposed touch this memory pool as well as creating PV lists
(cmd mempool or just temporarily created new pool needs to be used here.)

If you hold only read lock, there should be no need to modify vg structure.


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