On 06/15/2016 03:39 AM, Martin K. Petersen wrote:
>>>>>> "Hannes" == Hannes Reinecke <hare@xxxxxxx> writes:
>
> Hannes> Well, the primary issue is that 'blk_cloned_rq_check_limits()'
> Hannes> doesn't check for BLOCK_PC,
>
> Yes it does. It calls blk_rq_get_max_sectors() which has an explicit
> check for this:
>
> static inline unsigned int blk_rq_get_max_sectors(struct request *rq)
> {
> struct request_queue *q = rq->q;
>
> if (unlikely(rq->cmd_type != REQ_TYPE_FS))
> return q->limits.max_hw_sectors;
> [...]
>
> Hannes> The max_segments count, OTOH, _might_ change during failover
> Hannes> (different hardware has different max_segments setting, and this
> Hannes> is being changed during sg mapping), so there is some value to
> Hannes> be had from testing it here.
>
> Oh, this happens during failover? Are you sure it's not because DM is
> temporarily resetting the queue limits? max_sectors is going to be a
> single page in that case. I just discussed a backport regression in this
> department with Mike at LSF/MM. But that was for an older kernel.
>
> Accidentally resetting the limits during table swaps has happened a
> couple of times over the years. We trip it instantly with the database
> in failover testing.
>
Unfortunately, we have two distinct bugs lurking in that function.
The resetting limits during failover is something we've probably hitting
with a mixed initiator setting, but it will typically materialize as a
transient error when tripping over the _other_ check.
But this particular issue is seen directly during booting.
And as I've mentioned before: what is the purpose of this check?
'max_sectors' and 'max_hw_sectors' are checked during request assembly,
and those limits are not changed even after calling
blk_recalc_rq_segments(). And if we go over any device-imposed
restrictions we'll be getting an I/O error from the driver anyway.
So why have it at all?
Especially as the system boots happily with this check removed...
Cheers,
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@xxxxxxx +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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