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Date:   Mon, 21 May 2018 12:09:14 -0400
From:   Mike Snitzer <snitzer@redhat.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Kent Overstreet <kent.overstreet@gmail.com>,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
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Subject: Re: [PATCH 00/13] convert block layer to bioset_init()/mempool_init()
Message-ID: <20180521160907.GA19553@redhat.com>
References: <20180521140348.GA19069@redhat.com>
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On Mon, May 21 2018 at 11:36am -0400,
Jens Axboe <axboe@kernel.dk> wrote:

> On 5/21/18 9:18 AM, Mike Snitzer wrote:
> > On Mon, May 21 2018 at 11:09am -0400,
> > Jens Axboe <axboe@kernel.dk> wrote:
> > 
> >> On 5/21/18 9:04 AM, Mike Snitzer wrote:
> >>> On Mon, May 21 2018 at 10:52am -0400,
> >>> Jens Axboe <axboe@kernel.dk> wrote:
> >>>
...
> >>>> IMHO you're making a big deal out of something that should not be.
> >>>
> >>> I raised an issue that had seemingly not been considered at all.  Not
> >>> making a big deal.  Raising it for others' benefit.
> >>>
> >>>> If the dm bits are that sensitive and cache line honed to perfection
> >>>> already due to previous regressions in that area, then it might
> >>>> not be a bad idea to have some compile checks for false cacheline
> >>>> sharing between sensitive members, or spilling of a sub-struct
> >>>> into multiple cachelines.
> >>>>
> >>>> It's not like this was pushed behind your back. It's posted for
> >>>> review. It's quite possible the net change is a win for dm. Let's
> >>>> focus on getting it reviewed, rather than pontificate on what
> >>>> could potentially go all wrong with this.
> >>>
> >>> Why are you making this personal?  Or purely about DM?  I'm merely
> >>> pointing out this change isn't something that can be given a quick
> >>> blanket "looks good".
> >>
> >> I'm not making this personal at all?! You raised a (valid) concern,
> >> I'm merely stating why I don't think it's a high risk issue. I'm
> >> assuming your worry is related to dm, as those are the reports
> >> that would ultimately land on your desk.
> > 
> > Then we'll just agree to disagree with what this implies: "It's not like
> > this was pushed behind your back."
> 
> I'm afraid you've lost me now - it was not pushed behind your back,
> it was posted for review, with you on the CC list. Not trying to
> be deliberately dense here, I just don't see what our disagreement is.

You're having an off day ;)  Mondays and all?

I just raised an alignment concern that needs to be considered during
review by all stakeholders.  Wasn't upset at all.  Maybe my email tone
came off otherwise?

And then you got confused by me pointing out how it was weird for you to
suggest I felt this stuff was pushed behind my back.. and went on to
think I really think that.  It's almost like you're a confused hypnotist
seeding me with paranoid dilutions. ;)

/me waits for Jens to snap his fingers so he can just slowly step away
from this line of discussion that is solidly dead now...

> > Reality is I'm fine with the change.  Just think there is follow-on work
> > (now or later) that is needed.
> 
> It's not hard to run the quick struct layout checks or alignment. If
> there's a concern, that should be done now, instead of being deferred to
> later. There's no point merging something that we expect to have
> follow-on work. If that's the case, then it should not be merged. There
> are no dependencies in the patchset, except that the last patch
> obviously can't be applied until all of the previous ones are in.

Cool, sounds good.

> I took a quick look at the struct mapped_device layout, which I'm
> assuming is the most important on the dm side. It's pretty big to
> begin with, obviously this makes it bigger since we're now
> embedding the bio_sets. On my config, doesn't show any false sharing
> that would be problematic, or layout changes that would worry me. FWIW.

Great, thanks, do you happen to have a tree you can push so others can
get a quick compile and look at the series fully applied?

BTW, I'm upstream DM maintainer but I have a role in caring for related
subsystems (e.g. block core, etc) in downstream releases.. *cough* RHEL.
So this alignment concern wasn't ever purely about DM.

Mike