[DRBD-user] Strange message on Secondary (SyncSource) when i read a bloc on Primary (Synctarget)

Philipp Reisner philipp.reisner at linbit.com
Fri Oct 1 10:22:31 CEST 2004

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


On Thursday 30 September 2004 18:46, Lars Ellenberg wrote:
> / 2004-09-30 15:57:22 +0200
>
> \ Benoit.Ropartz at alcatel.fr:
> > Hello,
> >
> > DRBD 0.7
> > Kernel 2.4.x
> >
> > On Secondary (SyncSource) I see this message :
> >
> > drbd7: ASSERT( page_count(drbd_bio_get_page(&e->private_bio)) == 1 ) in
> > drbd_receiver.c:350
> >
> > The bloc is reading on Secondary because it is not yet syncing on Primary
> > !!!
> >
> > But the reading is OK (control by checksum....) !!!
> >
> > PS : each same reading give same editing on Secondary
>
> hm.
> you are not the first to report this.
> probably some thinko and the assert does not apply,
> rather than a bug  ...
>
> I guess Phillip will have a look at this before 0.7.5 is released?
>
> likely that for remote application reads this assert is simply wrong:
> of course the application has still a reference on that page.
>

No, It is a real issue! It is the zeroy_copy_IO network send that still
has the reference to that page. 

Under very rare conditions this could cause the read on the primary node
to return wrong data!

The race:  drbd_put_ee() puts this buffer onto our free list (nowadays
           at the end). If the receiver needs buffers it might get
           this one with drbd_get_ee() (unlikely since get_ee takes them
           from the beginning of the free list). And in case reding
           new data from the disk is quicker then the still ongoing
           zero-copy-IO network send ... we got it wrong.

Fixed now. 0.7.5 is really due...

-Philipp
-- 
: Dipl-Ing Philipp Reisner                      Tel +43-1-8178292-50 :
: LINBIT Information Technologies GmbH          Fax +43-1-8178292-82 :
: Schönbrunnerstr 244, 1120 Vienna, Austria    http://www.linbit.com :
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