Messages in this thread Patch in this message | | | Date | Wed, 25 Nov 2015 10:04:34 +0100 | From | Hannes Reinecke <> | Subject | Re: kernel BUG at drivers/scsi/scsi_lib.c:1096! |
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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. 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, which is evaluated via ->dm_dispatch_request() ->blk_insert_cloned_request() ->blk_rq_check_limits()
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?
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)
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