Re: block: don't check request size in blk_cloned_rq_check_limits()

[Hannes Reinecke <hare@suse.de>]

On 06/15/2016 03:39 AM, Martin K. Petersen wrote:
>>>>>> "Hannes" == Hannes Reinecke <hare@suse.de> 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
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