Re: elevator private data for REQ_FLUSH

[Jens Axboe]

On 2011-03-25 16:40, Mike Snitzer wrote:
> On Fri, Mar 25 2011 at 11:22am -0400,
> Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> 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?

It is, it forces a sort insert. I'll fix this up, I'm relieved we have a
good handle on this issue now.

-- 
Jens Axboe

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