linux-kernel - Re: [PATCH 05/13] blkcg: associate blkg when associating a device

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Date:   Mon, 3 Dec 2018 17:52:22 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Dennis Zhou <dennis@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Josef Bacik <josef@...icpanda.com>, kernel-team@...com,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/13] blkcg: associate blkg when associating a device

On Fri, Nov 30, 2018 at 01:54:26AM -0800, Christoph Hellwig wrote:
> > diff --git a/include/linux/bio.h b/include/linux/bio.h
> > index 62715a5a4f32..8bc9d9b29fd3 100644
> > --- a/include/linux/bio.h
> > +++ b/include/linux/bio.h
> > @@ -486,6 +486,12 @@ extern unsigned int bvec_nr_vecs(unsigned short idx);
> >  extern const char *bio_devname(struct bio *bio, char *buffer);
> >  
> >  #define bio_set_dev(bio, bdev) 			\
> > +do {						\
> > +	bio_set_dev_only(bio, bdev);		\
> > +	bio_associate_blkg(bio);		\
> > +} while (0)
> > +
> > +#define bio_set_dev_only(bio, bdev)		\
> 
> This lacks any explanation on when you would use bio_set_dev_only or
> bio_set_dev.  Please document why we need both and why you'd choose or
> the other.

I realized after thinking about this more and checking more use cases
that it isn't as simple as swapping macro uses because many of the
callers share common bio allocation paths. I think the simplest way
forward is to have writeback and swap do reassociation and split out bio
init code in a future series. So in v5, there is only bio_set_dev().

Thanks,
Dennis