Re: elevator private data for REQ_FLUSH

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Mar 25 2011 at 11:22am -0400,
Markus Trippelsdorf <markus@trippelsdorf.de> wrote:

> On 2011.03.25 at 17:15 +0200, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > Commit
> >     9d5a4e946ce5352f19400b6370f4cd8e72806278
> >     block: skip elevator data initialization for flush requests
> >     
> >     Skip elevator initialization for flush requests by passing priv=0 to
> >     blk_alloc_request() in get_request().  As such elv_set_request() is
> >     never called for flush requests.
> > 
> > introduced priv flag, to skip elevator_private data init for FLUSH requests.
> > This, I guess, lead to NULL pointer deref on my machine in cfq_insert_request,
> > which requires elevator_private to be set:
> > 
> >   1 [   78.982169] Call Trace:                                                                                                                                                                                                     
> >   2 [   78.982178]  [<ffffffff8122d1fe>] cfq_insert_request+0x4e/0x47d
> >   3 [   78.982184]  [<ffffffff8123e139>] ? do_raw_spin_lock+0x6b/0x122
> 
> > Should we in that case use ELEVATOR_INSERT_FLUSH for REQ_FLUSH | REQ_FUA requests
> > (like below)?
> > 
> > ---
> > 
> >  block/elevator.c |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > diff --git a/block/elevator.c b/block/elevator.c
> > index c387d31..b17e577 100644
> > --- a/block/elevator.c
> > +++ b/block/elevator.c
> > @@ -734,6 +734,8 @@ void __elv_add_request(struct request_queue *q, struct request *rq, int where)
> >  			q->end_sector = rq_end_sector(rq);
> >  			q->boundary_rq = rq;
> >  		}
> > +	} else if (rq->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
> > +		where = ELEVATOR_INSERT_FLUSH;
> >  	} else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
> >  		    where == ELEVATOR_INSERT_SORT)
> >  		where = ELEVATOR_INSERT_BACK;
> 
> Thanks. That solves all (corruption-) problems that I reported earlier in an other
> thread. 

So the flush-merge changes introduced ELEVATOR_INSERT_FLUSH (via commit
ae1b1539).  And the flush bio will now get ELEVATOR_INSERT_FLUSH in
__make_request().

So it is interesting that the flush is getting inserted in the elevator
at all.  AFAIK that shouldn't be (and historically hasn't been) the
case.

Combination of onstack plug changes?

  1 [   78.982169] Call Trace:
  2 [   78.982178]  [<ffffffff8122d1fe>] cfq_insert_request+0x4e/0x47d
  3 [   78.982184]  [<ffffffff8123e139>] ? do_raw_spin_lock+0x6b/0x122
  4 [   78.982189]  [<ffffffff81218c4a>] elv_insert+0x212/0x265
  5 [   78.982192]  [<ffffffff81218ced>] __elv_add_request+0x50/0x52
  6 [   78.982195]  [<ffffffff8121b44a>] flush_plug_list+0xce/0x12f
  7 [   78.982199]  [<ffffffff8121b4c0>] __blk_flush_plug+0x15/0x21
  8 [   78.982205]  [<ffffffff8146b321>] schedule+0x43e/0xbea

Mike