Re: kernel BUG at drivers/scsi/scsi_lib.c:1096!

[Takashi Iwai]

On Fri, 04 Dec 2015 18:02:58 +0100,
Jens Axboe wrote:
> 
> On 12/04/2015 09:59 AM, Takashi Iwai wrote:
> > On Wed, 25 Nov 2015 20:01:47 +0100,
> > Hannes Reinecke wrote:
> >>
> >> On 11/25/2015 07:01 PM, Mike Snitzer wrote:
> >>> On Wed, Nov 25 2015 at  4:04am -0500,
> >>> Hannes Reinecke <hare@xxxxxxx> 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 :-)
> >
> > Do we have already a fix?  Right now I got (likely) this kernel BUG()
> > on the almost latest Linus tree (commit 25364a9e54fb8296).  It
> > happened while I started a KVM right after a fresh boot.  The machine
> > paniced even before that, so I hit this twice today.
> 
> Update to the tree as-of yesterday (or today) and it should work. 
> 25364a9e54fb8296 doesn't include the latest block fixes that were sent 
> in yesterday, that should fix it. You need commit a88d32af18b8 or newer.

Alright, I'll give it a try.  Thanks for information!


Takashi
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