Messages in this thread | | | Date | Fri, 25 Mar 2011 20:50:26 +0100 | From | Jens Axboe <> | Subject | Re: elevator private data for REQ_FLUSH |
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On 2011-03-25 19:54, Mike Snitzer wrote: > On Fri, Mar 25 2011 at 11:50am -0400, > Jens Axboe <axboe@kernel.dk> wrote: > >> On 2011-03-25 16:40, Mike Snitzer wrote: >>> 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? >> >> It is, it forces a sort insert. I'll fix this up, I'm relieved we have a >> good handle on this issue now. > > Should we also add a safety net to avoid the potential for future silent > corruption, etc? E.g.:
Yes, I was thinking about something like that. I consider the patch merged an immediate stop gap, we need to improve this situation. It's not exactly pretty to have this sort of condition in both __make_request() and flush_plug_list(). Clearly it should be handled further down.
-- Jens Axboe
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