Messages in this thread | | | Subject | Re: kernel BUG at drivers/scsi/scsi_lib.c:1096! | From | Hannes Reinecke <> | Date | Wed, 25 Nov 2015 20:01:47 +0100 |
| |
On 11/25/2015 07:01 PM, Mike Snitzer wrote: > On Wed, Nov 25 2015 at 4:04am -0500, > Hannes Reinecke <hare@suse.de> wrote: > >> On 11/20/2015 04:28 PM, Ewan Milne wrote: >>> On Fri, 2015-11-20 at 15:55 +0100, Hannes Reinecke wrote: >>>> Can't we have a joint effort here? >>>> I've been spending a _LOT_ of time trying to debug things here, but >>>> none of the ideas I've come up with have been able to fix anything. >>> >>> Yes. I'm not the one primarily looking at it, and we don't have a >>> reproducer in-house. We just have the one dump right now. >>> >>>> >>>> I'm almost tempted to increase the count from scsi_alloc_sgtable() >>>> by one and be done with ... >>>> >>> >>> That might not fix it if it is a problem with the merge code, though. >>> >> And indeed, it doesn't. > > How did you arrive at that? Do you have a reproducer now? > Not a reproducer, but several dumps for analysis.
>> Seems I finally found the culprit. >> >> What happens is this: >> We have two paths, with these seg_boundary_masks: >> >> path-1: seg_boundary_mask = 65535, >> path-2: seg_boundary_mask = 4294967295, >> >> consequently the DM request queue has this: >> >> md-1: seg_boundary_mask = 65535, >> >> What happens now is that a request is being formatted, and sent >> to path 2. During submission req->nr_phys_segments is formatted >> with the limits of path 2, arriving at a count of 3. >> Now the request gets retried on path 1, but as the NOMERGE request >> flag is set req->nr_phys_segments is never updated. >> But blk_rq_map_sg() ignores all counters, and just uses the >> bi_vec directly, resulting in a count of 4 -> boom. >> >> So the culprit here is the NOMERGE flag, > > NOMERGE is always set in __blk_rq_prep_clone() for cloned requests. > Yes.
>> which is evaluated via >> ->dm_dispatch_request() >> ->blk_insert_cloned_request() >> ->blk_rq_check_limits() > > blk_insert_cloned_request() is the only caller of blk_rq_check_limits(); > anyway after reading your mail I'm still left wondering if your proposed > patch is correct. > >> If the above assessment is correct, the following patch should >> fix it: >> >> diff --git a/block/blk-core.c b/block/blk-core.c >> index 801ced7..12cccd6 100644 >> --- a/block/blk-core.c >> +++ b/block/blk-core.c >> @@ -1928,7 +1928,7 @@ EXPORT_SYMBOL(submit_bio); >> */ >> int blk_rq_check_limits(struct request_queue *q, struct request *rq) >> { >> - if (!rq_mergeable(rq)) >> + if (rq->cmd_type != REQ_TYPE_FS) >> return 0; >> >> if (blk_rq_sectors(rq) > blk_queue_get_max_sectors(q, >> rq->cmd_flags)) { >> >> >> Mike? Jens? >> Can you comment on it? > > You're not explaining the actual change in the patch very well; I think > you're correct but you're leaving the justification as an exercise to > the reviewer: > > blk_rq_check_limits() will call blk_recalc_rq_segments() after the > !rq_mergeable() check but you're saying for this case in question we > never get there -- due to the cloned request having NOMERGE set. > > So in blk_rq_check_limits() you've unrolled rq_mergeable() and > open-coded the lone remaining check (rq->cmd_type != REQ_TYPE_FS) > > I agree that the (rq->cmd_flags & REQ_NOMERGE_FLAGS) check in > the blk_insert_cloned_request() call-chain (via rq_mergeable()) makes no > sense for cloned requests that always have NOMERGE set. > > So you're saying that by having blk_rq_check_limits() go on to call > blk_recalc_rq_segments() this bug will be fixed? > That is the idea.
I've already established that in all instances I have seen so far req->nr_phys_segments is _less_ than req->bio->bi_phys_segments.
As it turns out, req->nr_phys_segemnts _would_ have been updated in blk_rq_check_limits(), but isn't due to the NOMERGE flag being set for the cloned request. So each cloned request inherits the values from the original request, despite the fact that req->nr_phys_segments _has_ to be evaluated in the final request_queue context, as the queue limits _might_ be different from the original (merged) queue limits of the multipath request queue.
> BTW, I think blk_rq_check_limits()'s export should be removed and the > function made static and renamed to blk_clone_rq_check_limits(), again: > blk_insert_cloned_request() is the only caller of blk_rq_check_limits() > Actually, seeing Jens' last comment the check for REQ_TYPE_FS is pointless, too, so we might as well remove the entire if-clause.
> Seems prudent to make that change now to be clear that this code is only > used by cloned requests. > Yeah, that would make sense. I'll be preparing a patch. With a more detailed description :-)
Cheers,
Hannes
-- Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)
| |