On Wed, Nov 25 2015 at 3:23pm -0500,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> On Wed, Nov 25 2015 at 2:24pm -0500,
> Jens Axboe <axboe@xxxxxx> wrote:
>
> > On 11/25/2015 12:10 PM, Hannes Reinecke wrote:
> > >The problem is that NOMERGE does too much, as it inhibits _any_ merging.
> >
> > Right, that is the point of the flag from the block layer view,
> > where it was originally added for the case mentioned.
>
> And we really don't want _any_ merging. The merging, if any, will have
> already happened in upper DM-multipath's elevator. So there should be
> no need to have the underlying SCSI paths do any merging.
>
> > >Unfortunately, the req->nr_phys_segments value is evaluated in the final
> > >_driver_ context _after_ the merging happend; cf
> > >scsi_lib.c:scsi_init_sgtable().
> > >As nr_phys_segments is inherited from the original request (and never
> > >recalculated with the new request queue limits) the following
> > >blk_rq_map_sg() call might end up at a different calculation, especially
> > >after retrying a request on another path.
> >
> > That all sounds pretty horrible. Why is blk_rq_check_limits()
> > checking for mergeable at all? If merging is disabled on the
> > request, I'm assuming that's an attempt at an optimization since we
> > know it won't change. But that should be tracked separately, like
> > how it's done on the bio.
>
> Not clear to me why it was checking for merging...
Ewan pointed out that blk_rq_check_limits()'s call to rq_mergable() was
introduced as part of Martin's DISCARD cleanup that prepared for
WRITE_SAME, see: commit e2a60da74 ("block: Clean up special command handling logic")
And prior to that, blk_rq_check_limits()'s (rq->cmd_flags & REQ_DISCARD)
check was introduced by some guy crazy doppelganger name "Ike Snitzer",
see commit: 3383977f ("block: update request stacking methods to support discards")
So basically we probably never needed the extra check in
blk_rq_check_limits() to begin with.. Ike was probably paranoid. ;)
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