Re: [PATCH v2 2/2] blk-throttle: fix wrong initialization in case of dm device

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Nov 20 2017 at  4:23pm -0500,
Shaohua Li <shli@kernel.org> wrote:

> On Mon, Nov 20, 2017 at 04:02:03PM -0500, Mike Snitzer wrote:
> > On Sun, Nov 19, 2017 at 9:00 PM, Joseph Qi <jiangqi903@gmail.com> wrote:
> > > From: Joseph Qi <qijiang.qj@alibaba-inc.com>
> > >
> > > dm device set QUEUE_FLAG_NONROT in resume, which is after register
> > > queue. That is to mean, the previous initialization in
> > > blk_throtl_register_queue is wrong in this case.
> > > Fix it by checking and then updating the info during root tg
> > > initialization as we don't have a better choice.
> > 
> > Given DM motivated this change, curious why you didn't send this to dm-devel?
> > 
> > In any case, not sure why you need to reference "resume".  Very few
> > people will appreciate that detail.
> > 
> > Better to just say: DM device sets QUEUE_FLAG_NONROT after the queue
> > is registered.
> > 
> > > Signed-off-by: Joseph Qi <qijiang.qj@alibaba-inc.com>
> > > ---
> > >  block/blk-throttle.c | 17 +++++++++++++++++
> > >  1 file changed, 17 insertions(+)
> > >
> > > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > > index bf52035..6d6b220 100644
> > > --- a/block/blk-throttle.c
> > > +++ b/block/blk-throttle.c
> > > @@ -541,6 +541,23 @@ static void throtl_pd_init(struct blkg_policy_data *pd)
> > >         if (cgroup_subsys_on_dfl(io_cgrp_subsys) && blkg->parent)
> > >                 sq->parent_sq = &blkg_to_tg(blkg->parent)->service_queue;
> > >         tg->td = td;
> > > +
> > > +       /*
> > > +        * dm device set QUEUE_FLAG_NONROT in resume, which is after resister
> > > +        * queue, so the previous initialization is wrong in this case. Check
> > > +        * and update it here.
> > > +        */
> > 
> > typo: s/resister/register/
> > But again, best to say:
> > DM device sets QUEUE_FLAG_NONROT after the queue is registered, ...
> > 
> > Also, an alternative to your patch would be to have DM's
> > dm_table_set_restrictions() call a blk-throttle function to setup
> > blk-throttle after it is done setting queue flags?
> > Saves all other drivers from having to run this 2 stage initialization
> > code.. just a thought.
> 
> Though the patch is a workaround, I'd prefer limiting the code into
> blk-throttle itself. Populating it to drivers is a layer violation to me. The
> initialization only runs once, so I think it's ok.

I fail to see how this is a "workaround".  Whatever you call it, the
blk-throttle code needs to cope with the fact that DM's request_queue
setup is different than other block devices.

If having DM call a function that blk-throttle provides (code that is in
block/blk-throttle.c) I fail to see the layer violation.  No more a
layer violation than any of the other block core code that block drivers
call to setup the request_queue's capabilities.

But I'm not well-versed on blk-throttle code (hence DM coming up lacking
relative to it).  SO I'll defer to you guys to sort it out how you think
best.

Thanks,
Mike