[Drbd-dev] drbd 8.4.3: refcounter overflow on re-sync

Marc Schiffbauer m at sys4.de
Fri Sep 19 17:16:53 CEST 2014

* Lars Ellenberg schrieb am 19.09.14 um 16:48 Uhr:
>On Fri, Sep 19, 2014 at 11:49:09AM +0200, Marc Schiffbauer wrote:
>> Hi,
>If you resolve that to a code line,
>I may be able to figure out what PAX is talking about.
>But from this stack trace alone, I have absolutely no idea what PAX
>is trying to say, which refcount could possibly be meant there,
>let alone why it could possibly overflow or.
>Ah, ok. Looking at [1], "PaX Team" says:
>| after having looked at the drbd code a bit i think this could be a
>| real bug in drbd but only upstream can tell for sure so you'll have to
>| contact them. you can show them the following that i figured out so far:
>| the refcount overflow was detected in
>| drivers/block/drbd/drbd_bitmap.c:bm_page_io_async at the
>| atomic_add(len >> 9, &mdev->rs_sect_ev)
>Well, yes, why would it not overflow.
>It is *not* a refcount.
>It is an atomic counter.
>It is meant to overflow.

Ok, then I can report this back and it should be fixed in PaX as a 
false postive. Thanks for clarifying this.

>| statement. rs_sect_ev is an atomic_t in struct drbd_conf declared in
>| drivers/block/drbd/drbd_int.h (i'll note here that i think the
>| rs_sect_in field is simiarly affected by this problem).
>| based on the code, these two fields don't look like refcounts, nor are
>| they free-running counters or statistics either (the usual cases for
>| false positives). instead they're some sector counts that get reset on
>| certain events (the details of which i can't tell as i don't know the
>| drbd code). therefore my feeling is that these counts are not supposed
>| to overflow as they'd otherwise lead to incorrect calculations in
>| drbd_rs_should_slow_down and drbd_rs_controller (the latter reads
>| rs_sect_in into an unsigned int btw, this is mixing up signed/unsigned
>| integers, that can't be good...).
>But yes, it *is* a "free running counter".
>For IO that has to be accounted to the resyncer.
>They are reset whenever a new resync starts.
>Apparently you are syncing more than 2*41 byte.
>That's ok.  Others do too.
>Having a lot of storage is no reason to drop the connection.


>| so what happened to you is that somehow rs_sect_ev reached 2G (that
>| corresponds to about 1TB of traffic between two counter resets or
>| 'events') and the signed overflow detection triggered on it (if that's
>| too unrealistic traffic for drbd then there was some other problem
>| calculating the sector counts that resulted in some big enough value to
>| trigger a signed overflow, though at the moment of the overflow 'len'
>| had a value of 8 only). in any case it looks that an atomic_t is not
>| enough to store real life sector counts and will have to be enlarged
>| probably (or the counters will have to be reset more frequently).
>It is perfectly ok for rs_sect_ev to overflow, it is meant to overflow.
>It is used in some signed modulo 32bit calculation only.
>> [63999.116913]  [<ffffffffa006f617>] ? drbd_thread_setup+0x4e/0x117 [drbd]
>> [63999.116917]  [<ffffffffa006f5c9>] ? conn_destroy+0x86/0x86 [drbd]
>> [63999.116922]  [<ffffffff8107fbfc>] ? kthread+0xd5/0xdd
>> [63999.116924]  [<ffffffff8107fb27>] ? kthread_worker_fn+0xf9/0xf9
>> [63999.116929]  [<ffffffff81535f74>] ? ret_from_fork+0x74/0xa0
>> [63999.116930]  [<ffffffff8107fb27>] ? kthread_worker_fn+0xf9/0xf9
>> [63999.116931] Code: 48 89 de 4c 89 ff e8 c3 80 00 00 85 c0 0f 85 b2 00 00 00 8b 54 24 1c f0 41 01 97 d0 04 00 00 71 0a f0 41 29 97 d0 04 00 00 cd 04 <bb> 03 00 00 00 f0 41 ff 87 24 02 00 00 71 0a f0 41 ff 8f 24 02
>> and drbd itself says:
>> [63999.116965] block drbd0: drbd_alloc_pages interrupted!
>Um, I'd guess it does say some things before that, too.
>Also, the other node will likely have something to say.
>Can you give some more context?
>Interrupted by whom?

by the PAX-System in the Kernel to protect the system becaus ethis 
*might* be an evil overflow.

>Is PAX delivering some signal?

I don't know, sorry :-/

>Why would it do that?

see above

>If so, stop it :-)

OK ;)

If you are still interested inmore context because you think there 
might be something wrong in the drbd code, then I can boot the 
previous kernel with PAX_REFCOUNT enabled to reproduce the proplem 

But it would be some effort to do zhis again, so I want to be sure 
its of any use for you now that it is clear that this must be a 
false positive.


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