linux-kernel - Re: [PATCH 06/11] drbd: use bio_clone_fast() instead of bio

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Date:   Fri, 21 Apr 2017 04:30:43 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     NeilBrown <neilb@...e.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Philipp Reisner <philipp.reisner@...bit.com>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        drbd-dev@...ts.linbit.com
Subject: Re: [PATCH 06/11] drbd: use bio_clone_fast() instead of bio_clone()

On Thu, Apr 20, 2017 at 03:38:49PM +1000, NeilBrown wrote:
> drbd does not modify the bi_io_vec of the cloned bio,
> so there is no need to clone that part.  So bio_clone_fast()
> is the better choice.
> For bio_clone_fast() we need to specify a bio_set.
> We could use fs_bio_set, which bio_clone() uses, or
> drbd_md_io_bio_set, which drbd uses for metadata, but it is
> generally best to avoid sharing bio_sets unless you can
> be certain that there are no interdependencies.
> 
> So create a new bio_set, drbd_io_bio_set, and use bio_clone_fast().
> 
> Signed-off-by: NeilBrown <neilb@...e.com>

All the zeroing of the gblobal variable looks unessecary / stupid.

But given that the surrounding code already does it:

Reviewed-by: Christoph Hellwig <hch@....de>

> +	bio = bio_clone_fast(bio_src, GFP_NOIO, drbd_io_bio_set); /* XXX cannot fail!! */

but maybe I'd drop this comment..