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

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Date:	Fri, 25 Mar 2011 16:22:28 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>, 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 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. 
-- 
Markus
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