linux-kernel - Re: [OOPS] elevator private data for REQ

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Date:	Fri, 25 Mar 2011 17:03:15 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel@...r.kernel.org, Mike Snitzer <snitzer@...hat.com>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [OOPS] elevator private data for REQ_FLUSH

On 2011.03.25 at 16:57 +0100, Jens Axboe wrote:
> On 2011-03-25 16:22, Markus Trippelsdorf 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. 
> 
> That's great. I'm surprised that this would cause silent corruption for
> you, should have been accompanied by an oops. Or was that with noop
> only?

The corruption happend during my bisection with cfq and also with noop
on the latest git kernel.

-- 
Markus
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