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

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Date:	Fri, 25 Mar 2011 16:57:16 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
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 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?


-- 
Jens Axboe

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