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

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jun 14 2016 at  9:39pm -0400,
Martin K. Petersen <martin.petersen@oracle.com> 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.

Not aware of any limits reset issue now...

> 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.

But feel free to throw your DB in failover tests (w/ dm-mpath) at a
recent kernel ;)